Decoder side motion vector refinement for affine motion model

ABSTRACT

An example device includes one or more processors configured determine a plurality of subblocks for a current block of video data. For each subblock, the one or more processors (a) generate initial motion vectors for a first prediction direction and a second prediction direction according to an affine motion model, and (b) determine, based on the initial motion vectors, a subblock bilateral matching cost for each respective offset among a plurality of offsets. For each respective offset, the one or more processors determine a respective summation of subblock bilateral matching costs. The one or more processors determine a lowest summation of subblock bilateral matching costs. The one or more processors select an offset associated with the lowest summation of subblock bilateral matching costs. The one or more processors modify the affine motion model based on the selected offset and code the current block based on the modified affine motion model.

This application claims the benefit of U.S. Provisional Application No.63/368,576, filed Jul. 15, 2022, the entire contents of which isincorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to video encoding and video decoding.

BACKGROUND

Digital video capabilities can be incorporated into a wide range ofdevices, including digital televisions, digital direct broadcastsystems, wireless broadcast systems, personal digital assistants (PDAs),laptop or desktop computers, tablet computers, e-book readers, digitalcameras, digital recording devices, digital media players, video gamingdevices, video game consoles, cellular or satellite radio telephones,so-called “smart phones,” video teleconferencing devices, videostreaming devices, and the like. Digital video devices implement videocoding techniques, such as those described in the standards defined byMPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced VideoCoding (AVC), ITU-T H.265/High Efficiency Video Coding (HEVC), ITU-TH.266/Versatile Video Coding (VVC), and extensions of such standards, aswell as proprietary video codecs/formats such as AOMedia Video 1 (AV1)that was developed by the Alliance for Open Media. The video devices maytransmit, receive, encode, decode, and/or store digital videoinformation more efficiently by implementing such video codingtechniques.

Video coding techniques include spatial (intra-picture) predictionand/or temporal (inter-picture) prediction to reduce or removeredundancy inherent in video sequences. For block-based video coding, avideo slice (e.g., a video picture or a portion of a video picture) maybe partitioned into video blocks, which may also be referred to ascoding tree units (CTUs), coding units (CUs) and/or coding nodes. Videoblocks in an intra-coded (I) slice of a picture are encoded usingspatial prediction with respect to reference samples in neighboringblocks in the same picture. Video blocks in an inter-coded (P or B)slice of a picture may use spatial prediction with respect to referencesamples in neighboring blocks in the same picture or temporal predictionwith respect to reference samples in other reference pictures. Picturesmay be referred to as frames, and reference pictures may be referred toas reference frames.

SUMMARY

In general, this disclosure describes techniques for inter prediction invideo coding. More specifically, this disclosure describes techniquesrelated to affine motion refinement, for example, by a video decoder.The techniques disclosed herein may be less complex and more practicalthan other techniques relating to decoder side motion vector refinement(DMVR) for a bi-directional predicted affine merge candidate. Forexample, the techniques of this disclosure may reduce the number oftimes a video coder generates a subblock motion field when performingDMVR for bi-directional predicted affine merge candidates This mayresult in easier implementation of the techniques of this disclosureand/or the saving of processing power.

In one example, a method includes determining a plurality of subblocksfor a current block of video data; for each subblock of the plurality ofsubblocks: (a) generating initial motion vectors, the initial motionvectors comprising at least one initial motion vector for a firstprediction direction and at least one initial motion vector for a secondprediction direction according to an affine motion model, and (b)determining, based on the initial motion vectors, a subblock bilateralmatching cost for each offset among a plurality of offsets; for eachrespective offset among the plurality of offsets, determining arespective summation of subblock bilateral matching costs associatedwith the plurality of subblocks thereby generating a plurality ofsummations of subblock bilateral matching costs; determining a lowestsummation of subblock bilateral matching costs from among the pluralityof summations of subblock bilateral matching costs; selecting an offsetfrom among the plurality of offsets associated with the lowest summationof subblock bilateral matching costs to determine a selected offset;modifying the affine motion model based on the selected offset togenerate a modified affine motion model; and coding the current blockbased on the modified affine motion model.

In another example, a device includes memory configured to store videodata and one or more processors implemented in circuitry andcommunicatively coupled to the memory, the one or more processors beingconfigured to: determine a plurality of subblocks for a current block ofthe video data; for each subblock of the plurality of subblocks: (a)generate initial motion vectors, the initial motion vectors comprisingat least one initial motion vector for a first prediction direction andat least one initial motion vector for a second prediction directionaccording to an affine motion model, and (b) determine, based on theinitial motion vectors, a subblock bilateral matching cost for eachoffset among a plurality of offsets; for each respective offset amongthe plurality of offsets, determine a respective summation of subblockbilateral matching costs associated with the plurality of subblocksthereby generating a plurality of summations of subblock bilateralmatching costs; determine a lowest summation of subblock bilateralmatching costs from among the plurality of summations of subblockbilateral matching costs; select an offset from among the plurality ofoffsets associated with the lowest summation of subblock bilateralmatching costs to determine a selected offset; modify the affine motionmodel based on the selected offset to generate a modified affine motionmodel; and code the current block based on the modified affine motionmodel.

In another example, a device includes: means for determining a pluralityof subblocks for a current block of the video data; means forgenerating, for each subblock of the plurality of subblocks, initialmotion vectors, the initial motion vectors comprising at least oneinitial motion vector for a first prediction direction and at least oneinitial motion vector for a second prediction direction according to anaffine motion model, and means for determining, based on the initialmotion vectors and for each subblock of the plurality of subblocks, asubblock bilateral matching cost for each offset among a plurality ofoffsets; means for determining, for each possible offset among theplurality of offsets, a respective summation of subblock bilateralmatching costs associated with the plurality of subblocks therebygenerating a plurality of summations of subblock bilateral matchingcosts; means for determining a lowest summation of subblock bilateralmatching costs from among the plurality of summations of subblockbilateral matching costs; means for selecting an offset from among theplurality of offsets associated with the lowest summation of subblockbilateral matching costs to determine a selected offset; means formodifying the affine motion model based on the selected offset togenerate a modified affine motion model; and means for coding thecurrent block based on the modified affine motion model.

In another example, a computer-readable storage medium is encoded withinstructions that, when executed, cause one or more programmableprocessors to: determine a plurality of subblocks for a current block ofvideo data; for each subblock of the plurality of subblocks: (a)generate initial motion vectors, the initial motion vectors comprisingat least one initial motion vector for a first prediction direction andat least one initial motion vector for a second prediction directionaccording to an affine motion model, and (b) determine, based on theinitial motion vectors, a subblock bilateral matching cost for eachoffset among a plurality of offsets; for each possible offset among theplurality of offsets, determine a respective summation of subblockbilateral matching costs associated with the plurality of subblocksthereby generating a plurality of summations of subblock bilateralmatching costs; determine a lowest summation of subblock bilateralmatching costs from among the plurality of summations of subblockbilateral matching costs; select an offset from among the plurality ofoffsets associated with the lowest summation of subblock bilateralmatching costs to determine a selected offset; modify the affine motionmodel based on the selected offset to generate a modified affine motionmodel; and code the current block based on the modified affine motionmodel.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system that may perform the techniques of this disclosure.

FIG. 2A is a conceptual diagram illustrating a control point based,6-parameter affine motion model.

FIG. 2B is a conceptual diagram illustrating a control point based,4-parameter affine motion model.

FIG. 3 is a conceptual diagram illustrating an example of bilateralmatching.

FIG. 4 is a block diagram illustrating an example video encoder that mayperform the techniques of this disclosure.

FIG. 5 is a block diagram illustrating an example video decoder that mayperform the techniques of this disclosure.

FIG. 6 is a flowchart illustrating example DMVR techniques according toone or more aspects of this disclosure.

FIG. 7 is a flowchart illustrating an example method for encoding acurrent block in accordance with the techniques of this disclosure.

FIG. 8 is a flowchart illustrating an example method for decoding acurrent block in accordance with the techniques of this disclosure.

DETAILED DESCRIPTION

Certain techniques for decoder side motion vector refinement (DMVR) fora bi-directional predicted affine merge candidate have added complexityto a video decoder implementing such techniques. This disclosuredescribes techniques that may be less complex and more practical thanthe certain techniques discussed above. This may result in easierimplementation of the techniques of this disclosure and/or the saving ofprocessing power. Some DMVR for bi-directional predicted affine mergecandidate techniques include generating subblock motion fields for eachsubblock of a block according to each of a plurality of offsets. Inother words, the techniques include generating a subblock motion fieldfor a subblock of the block for each of a plurality of possible offsetsand repeating this for each subblock of the block. The techniques ofthis disclosure may generate the motion fields of the each of thesubblocks once rather than as many times as there are possible offsets.As such, the techniques of this disclosure may greatly reduce the numberof times a video coder generates a subblock motion field when performingDMVR for bi-directional predicted affine merge candidates, therebysaving processing power and processing resources and/or extendingbattery charge.

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system 100 that may perform the techniques of this disclosure.The techniques of this disclosure are generally directed to coding(encoding and/or decoding) video data. In general, video data includesany data for processing a video. Thus, video data may include raw,unencoded video, encoded video, decoded (e.g., reconstructed) video, andvideo metadata, such as signaling data.

As shown in FIG. 1 , system 100 includes a source device 102 thatprovides encoded video data to be decoded and displayed by a destinationdevice 116, in this example. In particular, source device 102 providesthe video data to destination device 116 via a computer-readable medium110. Source device 102 and destination device 116 may comprise any of awide range of devices, including desktop computers, notebook (e.g.,laptop) computers, mobile devices, tablet computers, set-top boxes,telephone handsets such as smartphones, televisions, cameras, displaydevices, digital media players, video gaming consoles, video streamingdevice, broadcast receiver devices, or the like. In some cases, sourcedevice 102 and destination device 116 may be equipped for wirelesscommunication, and thus may be referred to as wireless communicationdevices.

In the example of FIG. 1 , source device 102 includes video source 104,memory 106, video encoder 200, and output interface 108. Destinationdevice 116 includes input interface 122, video decoder 300, memory 120,and display device 118. In accordance with this disclosure, videoencoder 200 of source device 102 and video decoder 300 of destinationdevice 116 may be configured to apply the techniques for affine motionrefinement. Thus, source device 102 represents an example of a videoencoding device, while destination device 116 represents an example of avideo decoding device. In other examples, a source device and adestination device may include other components or arrangements. Forexample, source device 102 may receive video data from an external videosource, such as an external camera. Likewise, destination device 116 mayinterface with an external display device, rather than include anintegrated display device.

System 100 as shown in FIG. 1 is merely one example. In general, anydigital video encoding and/or decoding device may perform techniques foraffine motion refinement. Source device 102 and destination device 116are merely examples of such coding devices in which source device 102generates coded video data for transmission to destination device 116.This disclosure refers to a “coding” device as a device that performscoding (encoding and/or decoding) of data. Thus, video encoder 200 andvideo decoder 300 represent examples of coding devices, in particular, avideo encoder and a video decoder, respectively. In some examples,source device 102 and destination device 116 may operate in asubstantially symmetrical manner such that each of source device 102 anddestination device 116 includes video encoding and decoding components.Hence, system 100 may support one-way or two-way video transmissionbetween source device 102 and destination device 116, e.g., for videostreaming, video playback, video broadcasting, or video telephony.

In general, video source 104 represents a source of video data (i.e.,raw, unencoded video data) and provides a sequential series of pictures(also referred to as “frames”) of the video data to video encoder 200,which encodes data for the pictures. Video source 104 of source device102 may include a video capture device, such as a video camera, a videoarchive containing previously captured raw video, and/or a video feedinterface to receive video from a video content provider. As a furtheralternative, video source 104 may generate computer graphics-based dataas the source video, or a combination of live video, archived video, andcomputer-generated video. In each case, video encoder 200 encodes thecaptured, pre-captured, or computer-generated video data. Video encoder200 may rearrange the pictures from the received order (sometimesreferred to as “display order”) into a coding order for coding. Videoencoder 200 may generate a bitstream including encoded video data.Source device 102 may then output the encoded video data via outputinterface 108 onto computer-readable medium 110 for reception and/orretrieval by, e.g., input interface 122 of destination device 116.

Memory 106 of source device 102 and memory 120 of destination device 116represent general purpose memories. In some examples, memories 106, 120may store raw video data, e.g., raw video from video source 104 and raw,decoded video data from video decoder 300. Additionally oralternatively, memories 106, 120 may store software instructionsexecutable by, e.g., video encoder 200 and video decoder 300,respectively. Although memory 106 and memory 120 are shown separatelyfrom video encoder 200 and video decoder 300 in this example, it shouldbe understood that video encoder 200 and video decoder 300 may alsoinclude internal memories for functionally similar or equivalentpurposes. Furthermore, memories 106, 120 may store encoded video data,e.g., output from video encoder 200 and input to video decoder 300. Insome examples, portions of memories 106, 120 may be allocated as one ormore video buffers, e.g., to store raw, decoded, and/or encoded videodata.

Computer-readable medium 110 may represent any type of medium or devicecapable of transporting the encoded video data from source device 102 todestination device 116. In one example, computer-readable medium 110represents a communication medium to enable source device 102 totransmit encoded video data directly to destination device 116 inreal-time, e.g., via a radio frequency network or computer-basednetwork. Output interface 108 may modulate a transmission signalincluding the encoded video data, and input interface 122 may demodulatethe received transmission signal, according to a communication standard,such as a wireless communication protocol. The communication medium maycomprise any wireless or wired communication medium, such as a radiofrequency (RF) spectrum or one or more physical transmission lines. Thecommunication medium may form part of a packet-based network, such as alocal area network, a wide-area network, or a global network such as theInternet. The communication medium may include routers, switches, basestations, or any other equipment that may be useful to facilitatecommunication from source device 102 to destination device 116.

In some examples, source device 102 may output encoded data from outputinterface 108 to storage device 112. Similarly, destination device 116may access encoded data from storage device 112 via input interface 122.Storage device 112 may include any of a variety of distributed orlocally accessed data storage media such as a hard drive, Blu-ray discs,DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or anyother suitable digital storage media for storing encoded video data.

In some examples, source device 102 may output encoded video data tofile server 114 or another intermediate storage device that may storethe encoded video data generated by source device 102. Destinationdevice 116 may access stored video data from file server 114 viastreaming or download.

File server 114 may be any type of server device capable of storingencoded video data and transmitting that encoded video data to thedestination device 116. File server 114 may represent a web server(e.g., for a website), a server configured to provide a file transferprotocol service (such as File Transfer Protocol (FTP) or File Deliveryover Unidirectional Transport (FLUTE) protocol), a content deliverynetwork (CDN) device, a hypertext transfer protocol (HTTP) server, aMultimedia Broadcast Multicast Service (MBMS) or Enhanced MBMS (eMBMS)server, and/or a network attached storage (NAS) device. File server 114may, additionally or alternatively, implement one or more HTTP streamingprotocols, such as Dynamic Adaptive Streaming over HTTP (DASH), HTTPLive Streaming (HLS), Real Time Streaming Protocol (RTSP), HTTP DynamicStreaming, or the like.

Destination device 116 may access encoded video data from file server114 through any standard data connection, including an Internetconnection. This may include a wireless channel (e.g., a Wi-Ficonnection), a wired connection (e.g., digital subscriber line (DSL),cable modem, etc.), or a combination of both that is suitable foraccessing encoded video data stored on file server 114. Input interface122 may be configured to operate according to any one or more of thevarious protocols discussed above for retrieving or receiving media datafrom file server 114, or other such protocols for retrieving media data.

Output interface 108 and input interface 122 may represent wirelesstransmitters/receivers, modems, wired networking components (e.g.,Ethernet cards), wireless communication components that operateaccording to any of a variety of IEEE 802.11 standards, or otherphysical components. In examples where output interface 108 and inputinterface 122 comprise wireless components, output interface 108 andinput interface 122 may be configured to transfer data, such as encodedvideo data, according to a cellular communication standard, such as 4G,4G-LTE (Long-Term Evolution), LTE Advanced, 5G, or the like. In someexamples where output interface 108 comprises a wireless transmitter,output interface 108 and input interface 122 may be configured totransfer data, such as encoded video data, according to other wirelessstandards, such as an IEEE 802.11 specification, an IEEE 802.15specification (e.g., ZigBee™), a Bluetooth™ standard, or the like. Insome examples, source device 102 and/or destination device 116 mayinclude respective system-on-a-chip (SoC) devices. For example, sourcedevice 102 may include an SoC device to perform the functionalityattributed to video encoder 200 and/or output interface 108, anddestination device 116 may include an SoC device to perform thefunctionality attributed to video decoder 300 and/or input interface122.

The techniques of this disclosure may be applied to video coding insupport of any of a variety of multimedia applications, such asover-the-air television broadcasts, cable television transmissions,satellite television transmissions, Internet streaming videotransmissions, such as dynamic adaptive streaming over HTTP (DASH),digital video that is encoded onto a data storage medium, decoding ofdigital video stored on a data storage medium, or other applications.

Input interface 122 of destination device 116 receives an encoded videobitstream from computer-readable medium 110 (e.g., a communicationmedium, storage device 112, file server 114, or the like). The encodedvideo bitstream may include signaling information defined by videoencoder 200, which is also used by video decoder 300, such as syntaxelements having values that describe characteristics and/or processingof video blocks or other coded units (e.g., slices, pictures, groups ofpictures, sequences, or the like). Display device 118 displays decodedpictures of the decoded video data to a user. Display device 118 mayrepresent any of a variety of display devices such as a liquid crystaldisplay (LCD), a plasma display, an organic light emitting diode (OLED)display, or another type of display device.

Although not shown in FIG. 1 , in some examples, video encoder 200 andvideo decoder 300 may each be integrated with an audio encoder and/oraudio decoder, and may include appropriate MUX-DEMUX units, or otherhardware and/or software, to handle multiplexed streams including bothaudio and video in a common data stream.

Video encoder 200 and video decoder 300 each may be implemented as anyof a variety of suitable encoder and/or decoder circuitry, such as oneor more microprocessors, digital signal processors (DSPs), applicationspecific integrated circuits (ASICs), field programmable gate arrays(FPGAs), discrete logic, software, hardware, firmware or anycombinations thereof. When the techniques are implemented partially insoftware, a device may store instructions for the software in asuitable, non-transitory computer-readable medium and execute theinstructions in hardware using one or more processors to perform thetechniques of this disclosure. Each of video encoder 200 and videodecoder 300 may be included in one or more encoders or decoders, eitherof which may be integrated as part of a combined encoder/decoder (CODEC)in a respective device. A device including video encoder 200 and/orvideo decoder 300 may comprise an integrated circuit, a microprocessor,and/or a wireless communication device, such as a cellular telephone.

Video encoder 200 and video decoder 300 may operate according to a videocoding standard, such as ITU-T H.265, also referred to as HighEfficiency Video Coding (HEVC) or extensions thereto, such as themulti-view and/or scalable video coding extensions. Alternatively, videoencoder 200 and video decoder 300 may operate according to otherproprietary or industry standards, such as ITU-T H.266, also referred toas Versatile Video Coding (VVC). In other examples, video encoder 200and video decoder 300 may operate according to a proprietary videocodec/format, such as AOMedia Video 1 (AV1), extensions of AV1, and/orsuccessor versions of AV1 (e.g., AV2). In other examples, video encoder200 and video decoder 300 may operate according to other proprietaryformats or industry standards. The techniques of this disclosure,however, are not limited to any particular coding standard or format. Ingeneral, video encoder 200 and video decoder 300 may be configured toperform the techniques of this disclosure in conjunction with any videocoding techniques that use affine motion refinement.

In general, video encoder 200 and video decoder 300 may performblock-based coding of pictures. The term “block” generally refers to astructure including data to be processed (e.g., encoded, decoded, orotherwise used in the encoding and/or decoding process). For example, ablock may include a two-dimensional matrix of samples of luminanceand/or chrominance data. In general, video encoder 200 and video decoder300 may code video data represented in a YUV (e.g., Y, Cb, Cr) format.That is, rather than coding red, green, and blue (RGB) data for samplesof a picture, video encoder 200 and video decoder 300 may code luminanceand chrominance components, where the chrominance components may includeboth red hue and blue hue chrominance components. In some examples,video encoder 200 converts received RGB formatted data to a YUVrepresentation prior to encoding, and video decoder 300 converts the YUVrepresentation to the RGB format. Alternatively, pre- andpost-processing units (not shown) may perform these conversions.

This disclosure may generally refer to coding (e.g., encoding anddecoding) of pictures to include the process of encoding or decodingdata of the picture. Similarly, this disclosure may refer to coding ofblocks of a picture to include the process of encoding or decoding datafor the blocks, e.g., prediction and/or residual coding. An encodedvideo bitstream generally includes a series of values for syntaxelements representative of coding decisions (e.g., coding modes) andpartitioning of pictures into blocks. Thus, references to coding apicture or a block should generally be understood as coding values forsyntax elements forming the picture or block.

HEVC defines various blocks, including coding units (CUs), predictionunits (PUs), and transform units (TUs). According to HEVC, a video coder(such as video encoder 200) partitions a coding tree unit (CTU) into CUsaccording to a quadtree structure. That is, the video coder partitionsCTUs and CUs into four equal, non-overlapping squares, and each node ofthe quadtree has either zero or four child nodes. Nodes without childnodes may be referred to as “leaf nodes,” and CUs of such leaf nodes mayinclude one or more PUs and/or one or more TUs. The video coder mayfurther partition PUs and TUs. For example, in HEVC, a residual quadtree(RQT) represents partitioning of TUs. In HEVC, PUs representinter-prediction data, while TUs represent residual data. CUs that areintra-predicted include intra-prediction information, such as anintra-mode indication.

As another example, video encoder 200 and video decoder 300 may beconfigured to operate according to VVC. According to VVC, a video coder(such as video encoder 200) partitions a picture into a plurality ofcoding tree units (CTUs). Video encoder 200 may partition a CTUaccording to a tree structure, such as a quadtree-binary tree (QTBT)structure or Multi-Type Tree (MTT) structure. The QTBT structure removesthe concepts of multiple partition types, such as the separation betweenCUs, PUs, and TUs of HEVC. A QTBT structure includes two levels: a firstlevel partitioned according to quadtree partitioning, and a second levelpartitioned according to binary tree partitioning. A root node of theQTBT structure corresponds to a CTU. Leaf nodes of the binary treescorrespond to coding units (CUs).

In an MTT partitioning structure, blocks may be partitioned using aquadtree (QT) partition, a binary tree (BT) partition, and one or moretypes of triple tree (TT) (also called ternary tree (TT)) partitions. Atriple or ternary tree partition is a partition where a block is splitinto three sub-blocks. In some examples, a triple or ternary treepartition divides a block into three sub-blocks without dividing theoriginal block through the center. The partitioning types in MTT (e.g.,QT, BT, and TT), may be symmetrical or asymmetrical.

When operating according to the AV1 codec, video encoder 200 and videodecoder 300 may be configured to code video data in blocks. In AV1, thelargest coding block that can be processed is called a superblock. InAV1, a superblock can be either 128×128 luma samples or 64×64 lumasamples. However, in successor video coding formats (e.g., AV2), asuperblock may be defined by different (e.g., larger) luma sample sizes.In some examples, a superblock is the top level of a block quadtree.Video encoder 200 may further partition a superblock into smaller codingblocks. Video encoder 200 may partition a superblock and other codingblocks into smaller blocks using square or non-square partitioning.Non-square blocks may include N/2×N, N×N/2, N/4×N, and N×N/4 blocks.Video encoder 200 and video decoder 300 may perform separate predictionand transform processes on each of the coding blocks.

AV1 also defines a tile of video data. A tile is a rectangular array ofsuperblocks that may be coded independently of other tiles. That is,video encoder 200 and video decoder 300 may encode and decode,respectively, coding blocks within a tile without using video data fromother tiles. However, video encoder 200 and video decoder 300 mayperform filtering across tile boundaries. Tiles may be uniform ornon-uniform in size. Tile-based coding may enable parallel processingand/or multi-threading for encoder and decoder implementations.

In some examples, video encoder 200 and video decoder 300 may use asingle QTBT or MTT structure to represent each of the luminance andchrominance components, while in other examples, video encoder 200 andvideo decoder 300 may use two or more QTBT or MTT structures, such asone QTBT/MTT structure for the luminance component and another QTBT/MTTstructure for both chrominance components (or two QTBT/MTT structuresfor respective chrominance components).

Video encoder 200 and video decoder 300 may be configured to usequadtree partitioning, QTBT partitioning, MTT partitioning, superblockpartitioning, or other partitioning structures.

In some examples, a CTU includes a coding tree block (CTB) of lumasamples, two corresponding CTBs of chroma samples of a picture that hasthree sample arrays, or a CTB of samples of a monochrome picture or apicture that is coded using three separate color planes and syntaxstructures used to code the samples. A CTB may be an N×N block ofsamples for some value of N such that the division of a component intoCTBs is a partitioning. A component is an array or single sample fromone of the three arrays (luma and two chroma) that compose a picture in4:2:0, 4:2:2, or 4:4:4 color format or the array or a single sample ofthe array that compose a picture in monochrome format. In some examples,a coding block is an M×N block of samples for some values of M and Nsuch that a division of a CTB into coding blocks is a partitioning.

The blocks (e.g., CTUs or CUs) may be grouped in various ways in apicture. As one example, a brick may refer to a rectangular region ofCTU rows within a particular tile in a picture. A tile may be arectangular region of CTUs within a particular tile column and aparticular tile row in a picture. A tile column refers to a rectangularregion of CTUs having a height equal to the height of the picture and awidth specified by syntax elements (e.g., such as in a picture parameterset). A tile row refers to a rectangular region of CTUs having a heightspecified by syntax elements (e.g., such as in a picture parameter set)and a width equal to the width of the picture.

In some examples, a tile may be partitioned into multiple bricks, eachof which may include one or more CTU rows within the tile. A tile thatis not partitioned into multiple bricks may also be referred to as abrick. However, a brick that is a true subset of a tile may not bereferred to as a tile. The bricks in a picture may also be arranged in aslice. A slice may be an integer number of bricks of a picture that maybe exclusively contained in a single network abstraction layer (NAL)unit. In some examples, a slice includes either a number of completetiles or only a consecutive sequence of complete bricks of one tile.

This disclosure may use “N×N” and “N by N” interchangeably to refer tothe sample dimensions of a block (such as a CU or other video block) interms of vertical and horizontal dimensions, e.g., 16×16 samples or 16by 16 samples. In general, a 16×16 CU will have 16 samples in a verticaldirection (y=16) and 16 samples in a horizontal direction (x=16).Likewise, an N×N CU generally has N samples in a vertical direction andN samples in a horizontal direction, where N represents a nonnegativeinteger value. The samples in a CU may be arranged in rows and columns.Moreover, CUs need not necessarily have the same number of samples inthe horizontal direction as in the vertical direction. For example, CUsmay comprise N×M samples, where M is not necessarily equal to N.

Video encoder 200 encodes video data for CUs representing predictionand/or residual information, and other information. The predictioninformation indicates how the CU is to be predicted in order to form aprediction block for the CU. The residual information generallyrepresents sample-by-sample differences between samples of the CU priorto encoding and the prediction block.

To predict a CU, video encoder 200 may generally form a prediction blockfor the CU through inter-prediction or intra-prediction.Inter-prediction generally refers to predicting the CU from data of apreviously coded picture, whereas intra-prediction generally refers topredicting the CU from previously coded data of the same picture. Toperform inter-prediction, video encoder 200 may generate the predictionblock using one or more motion vectors. Video encoder 200 may generallyperform a motion search to identify a reference block that closelymatches the CU, e.g., in terms of differences between the CU and thereference block. Video encoder 200 may calculate a difference metricusing a sum of absolute difference (SAD), sum of squared differences(SSD), mean absolute difference (MAD), mean squared differences (MSD),or other such difference calculations to determine whether a referenceblock closely matches the current CU. In some examples, video encoder200 may predict the current CU using uni-directional prediction orbi-directional prediction.

Some examples of VVC also provide an affine motion compensation mode,which may be considered an inter-prediction mode. In affine motioncompensation mode, video encoder 200 may determine two or more motionvectors that represent non-translational motion, such as zoom in or out,rotation, perspective motion, or other irregular motion types.

To perform intra-prediction, video encoder 200 may select anintra-prediction mode to generate the prediction block. Some examples ofVVC provide sixty-seven intra-prediction modes, including variousdirectional modes, as well as planar mode and DC mode. In general, videoencoder 200 selects an intra-prediction mode that describes neighboringsamples to a current block (e.g., a block of a CU) from which to predictsamples of the current block. Such samples may generally be above, aboveand to the left, or to the left of the current block in the same pictureas the current block, assuming video encoder 200 codes CTUs and CUs inraster scan order (left to right, top to bottom).

Video encoder 200 encodes data representing the prediction mode for acurrent block. For example, for inter-prediction modes, video encoder200 may encode data representing which of the various availableinter-prediction modes is used, as well as motion information for thecorresponding mode. For uni-directional or bi-directionalinter-prediction, for example, video encoder 200 may encode motionvectors using advanced motion vector prediction (AMVP) or merge mode.Video encoder 200 may use similar modes to encode motion vectors foraffine motion compensation mode.

AV1 includes two general techniques for encoding and decoding a codingblock of video data. The two general techniques are intra prediction(e.g., intra frame prediction or spatial prediction) and interprediction (e.g., inter frame prediction or temporal prediction). In thecontext of AV1, when predicting blocks of a current frame of video datausing an intra prediction mode, video encoder 200 and video decoder 300do not use video data from other frames of video data. For most intraprediction modes, video encoder 200 encodes blocks of a current framebased on the difference between sample values in the current block andpredicted values generated from reference samples in the same frame.Video encoder 200 determines predicted values generated from thereference samples based on the intra prediction mode.

Following prediction, such as intra-prediction or inter-prediction of ablock, video encoder 200 may calculate residual data for the block. Theresidual data, such as a residual block, represents sample by sampledifferences between the block and a prediction block for the block,formed using the corresponding prediction mode. Video encoder 200 mayapply one or more transforms to the residual block, to producetransformed data in a transform domain instead of the sample domain. Forexample, video encoder 200 may apply a discrete cosine transform (DCT),an integer transform, a wavelet transform, or a conceptually similartransform to residual video data. Additionally, video encoder 200 mayapply a secondary transform following the first transform, such as amode-dependent non-separable secondary transform (MDNSST), a signaldependent transform, a Karhunen-Loeve transform (KLT), or the like.Video encoder 200 produces transform coefficients following applicationof the one or more transforms.

As noted above, following any transforms to produce transformcoefficients, video encoder 200 may perform quantization of thetransform coefficients. Quantization generally refers to a process inwhich transform coefficients are quantized to possibly reduce the amountof data used to represent the transform coefficients, providing furthercompression. By performing the quantization process, video encoder 200may reduce the bit depth associated with some or all of the transformcoefficients. For example, video encoder 200 may round an n-bit valuedown to an m-bit value during quantization, where n is greater than m.In some examples, to perform quantization, video encoder 200 may performa bitwise right-shift of the value to be quantized.

Following quantization, video encoder 200 may scan the transformcoefficients, producing a one-dimensional vector from thetwo-dimensional matrix including the quantized transform coefficients.The scan may be designed to place higher energy (and therefore lowerfrequency) transform coefficients at the front of the vector and toplace lower energy (and therefore higher frequency) transformcoefficients at the back of the vector. In some examples, video encoder200 may utilize a predefined scan order to scan the quantized transformcoefficients to produce a serialized vector, and then entropy encode thequantized transform coefficients of the vector. In other examples, videoencoder 200 may perform an adaptive scan. After scanning the quantizedtransform coefficients to form the one-dimensional vector, video encoder200 may entropy encode the one-dimensional vector, e.g., according tocontext-adaptive binary arithmetic coding (CABAC). Video encoder 200 mayalso entropy encode values for syntax elements describing metadataassociated with the encoded video data for use by video decoder 300 indecoding the video data.

To perform CABAC, video encoder 200 may assign a context within acontext model to a symbol to be transmitted. The context may relate to,for example, whether neighboring values of the symbol are zero-valued ornot. The probability determination may be based on a context assigned tothe symbol.

Video encoder 200 may further generate syntax data, such as block-basedsyntax data, picture-based syntax data, and sequence-based syntax data,to video decoder 300, e.g., in a picture header, a block header, a sliceheader, or other syntax data, such as a sequence parameter set (SPS),picture parameter set (PPS), or video parameter set (VPS). Video decoder300 may likewise decode such syntax data to determine how to decodecorresponding video data.

In this manner, video encoder 200 may generate a bitstream includingencoded video data, e.g., syntax elements describing partitioning of apicture into blocks (e.g., CUs) and prediction and/or residualinformation for the blocks. Ultimately, video decoder 300 may receivethe bitstream and decode the encoded video data.

In general, video decoder 300 performs a reciprocal process to thatperformed by video encoder 200 to decode the encoded video data of thebitstream. For example, video decoder 300 may decode values for syntaxelements of the bitstream using CABAC in a manner substantially similarto, albeit reciprocal to, the CABAC encoding process of video encoder200. The syntax elements may define partitioning information forpartitioning of a picture into CTUs, and partitioning of each CTUaccording to a corresponding partition structure, such as a QTBTstructure, to define CUs of the CTU. The syntax elements may furtherdefine prediction and residual information for blocks (e.g., CUs) ofvideo data.

The residual information may be represented by, for example, quantizedtransform coefficients. Video decoder 300 may inverse quantize andinverse transform the quantized transform coefficients of a block toreproduce a residual block for the block. Video decoder 300 uses asignaled prediction mode (intra- or inter-prediction) and relatedprediction information (e.g., motion information for inter-prediction)to form a prediction block for the block. Video decoder 300 may thencombine the prediction block and the residual block (on asample-by-sample basis) to reproduce the original block. Video decoder300 may perform additional processing, such as performing a deblockingprocess to reduce visual artifacts along boundaries of the block. Thisdisclosure may generally refer to “signaling” certain information, suchas syntax elements. The term “signaling” may generally refer to thecommunication of values for syntax elements and/or other data used todecode encoded video data. That is, video encoder 200 may signal valuesfor syntax elements in the bitstream. In general, signaling refers togenerating a value in the bitstream. As noted above, source device 102may transport the bitstream to destination device 116 substantially inreal time, or not in real time, such as might occur when storing syntaxelements to storage device 112 for later retrieval by destination device116.

In accordance with the techniques of this disclosure, a video coder suchas video decoder 300 may divide a current block of the video data into aplurality of subblocks; generate respective initial motion vectors forboth prediction directions for each subblock according to an initialaffine motion model; determine a respective subblock bilateral matchingcost for each possible offset of a plurality of offsets for eachsubblock; determine a respective accumulated subblock bilateral matchingcost for each possible offset, each respective accumulated subblockbilateral matching cost corresponding to a respective bilateral matchingcost for the current block for one of the possible offsets; based on acorresponding bilateral matching cost for the current block having alowest bilateral matching cost of each respective accumulated subblockbilateral matching costs, select a possible offset of the possibleoffsets; and code the current block based on the selected possibleoffset.

Affine mode is now described. FIG. 2A is a conceptual diagramillustrating a control point based, 6-parameter affine motion model. Asshown FIG. 2A, the affine motion field of the block is described bymotion information of two control points (170A and 170B), also referredto as a 4-parameter model. FIG. 2B is a conceptual diagram illustratinga control point based, 4-parameter affine motion model. As shown FIG.6B, the affine motion field of the block is described by motioninformation of three control points (172A-172C) and three control pointmotion vectors, which is also referred to as a 6-parameter model.

For example, an affine motion model can be described as

$\left\{ \begin{matrix}{v_{x} = {{ax} + {by} + e}} \\{v_{y} = {{cx} + {dy} + f}}\end{matrix} \right.$

wherein (v_(x), v_(y)) is a motion vector at the coordinate (x,y), anda, b, c, d, e, and f are the six affine parameters. This affine motionmodel may be referred to as a 6-parameters affine motion model. In atypical video coder, which, in some examples may be represented by videoencoder 200 or video decoder 300, a picture is partitioned into blocksfor block-based coding. The affine motion model for a block can also bedescribed by the 3 motion vectors (MVs) {right arrow over (v)}₀(v_(0x),v_(0y)), {right arrow over (v)}₁=(v_(1x),v_(1y)), and {rightarrow over (v)}₂=(v_(2x),v_(2y)) at 3 different locations that are notall along a same line. The 3 locations are usually referred to ascontrol-points, the 3 motion vectors are referred to as control pointmotion vectors (CPMVs). In the case when the 3 control-points are at the3 corners of the block, the affine motion can be described as

$\left\{ \begin{matrix}{v_{x} = {{\frac{\left( {v_{1x} - v_{0x}} \right)}{blkW}x} + {\frac{\left( {v_{2x} - v_{0x}} \right)}{blkH}y} + v_{0x}}} \\{v_{y} = {{\frac{\left( {v_{1y} - v_{0y}} \right)}{blkW}x} + {\frac{\left( {v_{2y} - v_{0y}} \right)}{blkH}y} + v_{0y}}}\end{matrix} \right.$

wherein blkW and blkH are the width and height of the block.

In affine mode, different motion vectors can be derived for each pixelin the block according to the associated affine motion model. Therefore,motion compensation can be performed pixel-by-pixel. However, to reducethe complexity, subblock based motion compensation is usually adopted orutilized, wherein the block is partitioned into multiple subblocks (thateach have a smaller size than the block) and each subblock is associatedwith one motion vector for block-based motion compensation. For example,video encoder 200 or video decoder 300 may associate each subblock witha motion vector. The motion vector for each subblock may be derivedusing the representative coordinate of the subblock. Typically, thecenter position is used. In one example, the block is partitioned intonon-overlapping subblocks. The block width is blkW, block height isblkH, the subblock width is sbW and subblock height is sbH, then there'sblkH/sbH rows of subblocks and blkW/sbW subblocks in each row. For asix-parameter affine motion model, the motion vector for the subblock(referred to as subblock MV) at i_(th) row (0<=i<blkW/sbW) and j_(th)(0<=j<blkH/sbH) column is derived as

$\left\{ \begin{matrix}{v_{x} = {{\frac{\left( {v_{1x} - v_{0x}} \right)}{blkW}\left( {{j*{sbW}} + \frac{sbW}{2}} \right)} + {\frac{\left( {v_{2x} - v_{0x}} \right)}{blkH}\left( {{i*{sbH}} + \frac{sbH}{2}} \right)} + v_{0x}}} \\{v_{y} = {{\frac{\left( {v_{1y} - v_{0y}} \right)}{blkW}\left( {{j*{sbW}} + \frac{sbW}{2}} \right)} + {\frac{\left( {v_{2y} - v_{0y}} \right)}{blkH}\left( {{i*{sbH}} + \frac{sbH}{2}} \right)} + v_{0y}}}\end{matrix} \right.$

The subblock MVs may be rounded to the predefined precision and storedin the motion buffer for motion compensation and motion vectorprediction. For example, video encoder 200 or video decoder 300 mayround the subblock MVs.

A simplified 4-parameters affine model (for zoom and rotational motion)is described as

$\left\{ \begin{matrix}{v_{x} = {{ax} - {by} + e}} \\{v_{y} = {{bx} + {ay} + f}}\end{matrix} \right.$

Similarly, the 4-parameters affine model for a block can be described by2 CPMVs {right arrow over (v)}₀=(v_(0x),v_(0y)) and {right arrow over(v)}₁=(v_(1x), v_(1y)) at the 2 corners (typically top-left andtop-right) of the block. The motion field is then described as

$\left\{ \begin{matrix}{v_{x} = {{\frac{\left( {v_{1x} - v_{0x}} \right)}{blkW}x} - {\frac{\left( {v_{1y} - v_{0y}} \right)}{blkW}y} + v_{0x}}} \\{v_{y} = {{\frac{\left( {v_{1y} - v_{0y}} \right)}{blkW}x} + {\frac{\left( {v_{1x} - v_{0x}} \right)}{blkW}y} + v_{0y}}}\end{matrix} \right.$

The subblock MV at i_(th) row and j_(th) column is derived as

$\left\{ \begin{matrix}{v_{x} = {{\frac{\left( {v_{1x} - v_{0x}} \right)}{blkW}\left( {{j*{sbW}} + \frac{sbW}{2}} \right)} - {\frac{\left( {v_{1y} - v_{0y}} \right)}{blkW}\left( {{i*{sbH}} + \frac{sbH}{2}} \right)} + v_{0x}}} \\{v_{y} = {{\frac{\left( {v_{1y} - v_{0y}} \right)}{blkW}\left( {{j*{sbW}} + \frac{sbW}{2}} \right)} + {\frac{\left( {v_{1x} - v_{0x}} \right)}{blkW}\left( {{i*{sbH}} + \frac{sbH}{2}} \right)} + v_{0y}}}\end{matrix} \right.$

Prediction refinement for affine mode is now described. After thesub-block based affine motion compensation is performed, the predictionsignal can be refined by adding an offset derived based on thepixel-wise motion and the gradient of the prediction signal. Forexample, video encoder 200 or video decoder 300 may add the offset tothe prediction signal. The offset at location (m,n) can be calculatedas:

ΔI(m,n)=g _(x)(m,n)*Δv _(x)(m,n)+g _(y)(m,n)*Δv _(y)(m,n)

wherein g_(x)(m,n) is the horizontal gradient and g_(y)(m,n) is thevertical gradient of the prediction signal, respectively. Δv_(x)(m,n)and Δv_(y)(m,n) are the differences in the x and y components betweenthe motion vector calculated at location pixel location (m,n) and thesubblock MV. In an example, the coordinate of the top-left sample of thesubblock is (0,0), the center of the subblock is

$\left( {\frac{sbW}{2},\frac{sbH}{2}} \right).$

Given the affine motion parameters a, b, c, and d; Δv_(x) (m,n) andΔv_(y) (m,n) can be derived as:

${\Delta{v_{x}\left( {m,n} \right)}} = {{a*\left( {m - \frac{sbW}{2}} \right)} + {b*\left( {n - \frac{sbH}{2}} \right)}}$${\Delta{v_{y}\left( {m,n} \right)}} = {{c*\left( {m - \frac{sbW}{2}} \right)} + {d*\left( {n - \frac{sbH}{2}} \right)}}$

In the control-points-based affine motion model, the affine motionparameters a, b, c, and d may be calculated from the CPMVs as

$a = \frac{\left( {v_{1x} - v_{0x}} \right)}{blkW}$$b = \frac{\left( {v_{2x} - v_{0x}} \right)}{blkH}$$c = \frac{\left( {v_{1y} - v_{0y}} \right)}{blkW}$$d = \frac{\left( {v_{2y} - v_{0y}} \right)}{blkH}$

Decoder side motion vector refinement is now discussed. In the VersatileVideo Coding standard (VVC), bilateral-matching-based decoder sidemotion vector refinement (DMVR) is applied to increase the accuracy ofthe MVs of a bi-prediction merge candidate. Video decoder 300 may usethe bilateral-matching techniques to calculate the SAD between the twocandidate blocks in the reference picture list L0 and list L1.

FIG. 3 is a conceptual diagram illustrating an example of bilateralmatching. As illustrated in FIG. 3 , the SAD between the blocks 180 and182 based on each MV candidate around the initial MV is calculated. TheMV candidate with the lowest SAD becomes the refined MV and used togenerate the bi-predicted signal. The SAD of the initial MVs issubtracted by ¼ of the SAD value. The temporal distances (e.g., PictureOrder Count (POC) differences) from two reference pictures to thecurrent picture shall be the same (in some examples), therefore, theMVD0 is just the opposite sign of MVD1.

The refinement search range may be two integer luma samples from theinitial MV. The searching includes the integer sample offset searchstage and fractional sample refinement stage. A 25 points full searchmay be applied for integer sample offset searching. The SAD of theinitial MV pair (e.g., a MV in an L0 direction and an MV in an L1direction) is first calculated. If the SAD of the initial MV pair issmaller than a threshold, the integer sample stage of DMVR isterminated. Otherwise SADs of the remaining 24 points are calculated andchecked in raster scanning order. The point with the smallest SAD may beselected as the output of integer sample offset searching stage.

The integer sample search is followed by a fractional sample refinement.For example, video decoder 300 may perform a fractional samplerefinement. To save on calculational complexity, the fractional samplerefinement may be derived by using a parametric error surface equation,instead of additional search(es) with SAD comparison(s). The fractionalsample refinement may be conditionally invoked based on the output ofthe integer sample search stage. When the integer sample search stage isterminated with a center having the smallest SAD in either the firstiteration or the second iteration search, the fractional samplerefinement may be further applied.

In parametric error surface based sub-pixel offsets estimation, thecenter position cost and the costs at four neighboring positions fromthe center are used to fit a 2-D parabolic error surface equation of thefollowing form:

E(x,y)=A(x−x _(min))² +B(y−y _(min))² +C  (1)

where (x_(min), y_(min)) corresponds to the fractional position with theleast cost and C corresponds to the minimum cost value. By solving theabove equation by using the cost value of the five search points, the(x_(min), y_(min)) is computed as:

x _(min)=(E(−1,0)−E(1,0))/(2(E(−1,0)+E(1,0)−2E(0,0)))  (2)

y _(min)=(E(0,−1)−E(0,1))/(2((E(0,−1)+E(0,1)−2E(0,0)))  (3)

The value of x_(min) and y_(min) are automatically constrained to bebetween −8 and 8 since all cost values are positive and the smallestvalue is E(0,0). This corresponds to half pel offset with 1/16th-pel MVaccuracy in VVC. The computed fractional (x_(min), y_(min)) are added tothe integer distance refinement MV to get the sub-pixel accuraterefinement delta MV.

In VVC, the resolution of the MVs is 1/16 luma samples. The samples atthe fractional position are interpolated using an 8-tap interpolationfilter. For example, video decoder 300 may interpolate the samples atthe fractional position. In DMVR, the search points are surrounding theinitial fractional-pel MV with integer sample offset, therefore thesamples of those fractional position need to be interpolated for theDMVR search process. To reduce the calculation complexity, the bi-linearinterpolation filter may be used to generate the fractional samples forthe searching process in DMVR. Another important effect is that by usinga bi-linear filter with a 2-sample search range, the DVMR does notaccess more reference samples than the normal motion compensationprocess. After the refined MV is attained with DMVR search process, thenormal 8-tap interpolation filter may be applied to generate the finalprediction. In order to not access more reference samples than thenormal MC process, the samples, which are not needed for theinterpolation process based on the original MV but are needed for theinterpolation process based on the refined MV, may be padded from thoseavailable samples.

When the width and/or height of a CU are larger than 16 luma samples,the CU may be further split into subblocks with width and/or heightequal to 16 luma samples for the DMVR process.

In VVC, the DMVR process can be applied for the CUs which are coded withfollowing modes and/or features: a) CU level merge mode withbi-prediction MV; b) One reference picture is in the past and anotherreference picture is in the future with respect to the current picture;c) The distances (e.g., POC difference) from two reference pictures tothe current picture are same; d) Both reference pictures are short-termreference pictures; e) CU has more than 64 luma samples; 0 Both CUheight and CU width are larger than or equal to 8 luma samples; g) BCWweight index indicates equal weight; h) WP is not enabled for thecurrent block; and/or i) CIIP mode is not used for the current block.

Decoder side motion vector refinement for affine merge mode is nowdiscussed. In Chen, et al., “Non-EE2: DMVR for affine merge codedblocks,” Joint Video Experts Team (JVET) of ITU-T SG 16 WP 3 and ISO/IECJTC 1/SC 29/WG 11, 27th Meeting, by teleconference, 13-22 Jul. 2022,JVET-AA0144, decoder side motion vector refinement (DMVR) is proposedfor bi-directional predicted affine merge candidate. Video decoder 300may apply a translation MV offset to all the CPMVs of the candidate inthe affine merge list if the candidate meets the DMVR condition. And theMV offset may be derived by minimizing the cost of bilateral matching,which is the same as conventional DMVR. The MV offset searching processis the same as the first pass of multi-pass DMVR (prediction unit level)in ECM (Enhanced Compression Model). A 3×3 square search pattern may beused to loop through the search range [−8, +8] in horizontal directionand [−8, +8] in vertical direction to find the best integer MV offset. Ahalf pel search may be conducted around the best integer position and anerror surface estimation may be performed to find a MV offset with 1/16precision.

To calculate the bilateral matching cost for a given offset, videodecoder 300 may apply the following steps: a) the offset is first addedto each of the CPMVs in both directions; b) the subblock motion field isderived according to the updated CPMVs; c) subblock based motioncompensation is applied according to the subblock motion field in eachdirection; and d) prediction refinement for affine motion (as describedabove) is applied to refine the predictor generated from step c).

For example, video decoder 300 may add the offset to each of the CPMVsin both directions. Video decoder 300 may derive the subblock motionfield according to the updated CPMVs. Video decoder 300 may applysubblock-based motion compensation according to the subblock motionfield in each direction. Video decoder 300 may apply predictionrefinement for affine motion (as described above) to refine thepredictor generated from applying subblock-based motion compensationaccording to the subblock motion field in each direction. Suchtechniques may significantly increase the complexity of the DMVR for theaffine merge candidate.

According to the techniques of this disclosure, a less complex and morepractical DMVR design for an affine motion model is disclosed. This DMVRdesign can be summarized in the following steps: 1) Divide the currentblock into subblocks; 2) Generate initial motion vectors (of bothprediction directions) for each subblock (subblock motion fields)according to the initial affine motion model; 3) Loop over eachsubblock, calculate subblock bilateral matching cost for all possibleoffsets; 4) For each possible offset, accumulate the subblock bilateralmatching cost to generate the bilateral matching cost corresponding tothe entire block; and 5) Determine the best offset by selecting the onewith minimum bilateral matching cost corresponding to the entire block.

For example, video decoder 300 may divide the current block intosubblocks. Video decoder 300 may generate initial motion vectors (inboth prediction directions) for each subblock (e.g., subblock motionfields) according to the initial affine motion model. Video decoder 300may loop over each subblock, and calculate subblock bilateral matchingcosts for each of the possible offsets. Video decoder 300 may, for eachof the possible offsets, accumulate the subblock bilateral matching costof the possible offset across all of the subblocks of the block togenerate the bilateral matching cost corresponding to the entire block.Video decoder 300 may determine the best (e.g., lowest cost) offset byselecting the offset of the possible offsets with the lowest bilateralmatching cost corresponding to the entire block. For example, videodecoder 300 may select the offset corresponding to the lowestaccumulated subblock bilateral matching cost to use when refining apredictor. In this way, the subblock motion fields are generated onlyonce instead of for each candidate offset, which may reducecomputational complexity and save power.

The subblock size used in the above techniques may be the same as thatis used for affine motion compensation, typically 4×4. The subblock sizemay also be a different size. In some examples, the subblock size may bea larger subblock (8×8, 16×16, for example), which may result in a fewertotal number of subblocks in a current block, to reduce the complexityof the DMVR process.

In step 3), calculating subblock bilateral matching costs for allpossible offsets, given the offset and initial motion vectors (generatedin step 2)), the candidate motion vectors may be derived (in the case ofmirroring bilateral matching, the offset is added to the motion vectorin one direction and subtracted from the motion vector in the otherdirection), then motion compensation is performed to generate thepredictors for the corresponding subblock. For example, video decoder300 may derive candidate motion vectors and perform motion compensationto generate predictors for a subblock. In some examples, the predictionrefinement may be skipped for simplification purposes. Also, note thatinterpolation can be applied to generate predictors for all possibleoffsets in one step, which may significantly reduce computationalcomplexity. For example, video decoder 300 may apply interpolation togenerate samples which may not be at an integer positions. Bilinearinterpolation may be used instead of 8-tap (6-tap, or 12-tap)interpolation filters that are typically used for final motioncompensation.

Parametric error surface based sub-pixel offsets estimation can be alsoapplied after step 5) to generate a sub-pixel offset. For example, videodecoder 300 may estimate sub-pixel offsets using parametric errorsurface techniques.

Certain size constraint(s) can be applied. For example, video decoder300 may only apply DMVR to an affine block that is larger than N×N,wherein N is set equal to 8, 16, 32, or another integer. In anotherexample, the size constrain depends on the subblock size used in theDMVR process and N is set equal to the subblock size.

High level syntax may be used to control whether DMVR for the affineblock is applied, and such high level syntax may be signaled in thebitstream. For example, video encoder 200 may signal a syntax elementindicative of whether DMVR is used for an affine block in the sequenceparameter set, picture header, slice header, etc. Video decoder 300 mayparse the syntax element to determine whether to apply DMVR to an affineblock.

FIG. 4 is a block diagram illustrating an example video encoder 200 thatmay perform the techniques of this disclosure. FIG. 4 is provided forpurposes of explanation and should not be considered limiting of thetechniques as broadly exemplified and described in this disclosure. Forpurposes of explanation, this disclosure describes video encoder 200according to the techniques of VVC (ITU-T H.266, under development), andHEVC (ITU-T H.265). However, the techniques of this disclosure may beperformed by video encoding devices that are configured to other videocoding standards and video coding formats, such as AV1 and successors tothe AV1 video coding format.

In the example of FIG. 4 , video encoder 200 includes video data memory230, mode selection unit 202, residual generation unit 204, transformprocessing unit 206, quantization unit 208, inverse quantization unit210, inverse transform processing unit 212, reconstruction unit 214,filter unit 216, decoded picture buffer (DPB) 218, and entropy encodingunit 220. Any or all of video data memory 230, mode selection unit 202,residual generation unit 204, transform processing unit 206,quantization unit 208, inverse quantization unit 210, inverse transformprocessing unit 212, reconstruction unit 214, filter unit 216, DPB 218,and entropy encoding unit 220 may be implemented in one or moreprocessors or in processing circuitry. For instance, the units of videoencoder 200 may be implemented as one or more circuits or logic elementsas part of hardware circuitry, or as part of a processor, ASIC, or FPGA.Moreover, video encoder 200 may include additional or alternativeprocessors or processing circuitry to perform these and other functions.

Video data memory 230 may store video data to be encoded by thecomponents of video encoder 200. Video encoder 200 may receive the videodata stored in video data memory 230 from, for example, video source 104(FIG. 1 ). DPB 218 may act as a reference picture memory that storesreference video data for use in prediction of subsequent video data byvideo encoder 200. Video data memory 230 and DPB 218 may be formed byany of a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. Video datamemory 230 and DPB 218 may be provided by the same memory device orseparate memory devices. In various examples, video data memory 230 maybe on-chip with other components of video encoder 200, as illustrated,or off-chip relative to those components.

In this disclosure, reference to video data memory 230 should not beinterpreted as being limited to memory internal to video encoder 200,unless specifically described as such, or memory external to videoencoder 200, unless specifically described as such. Rather, reference tovideo data memory 230 should be understood as reference memory thatstores video data that video encoder 200 receives for encoding (e.g.,video data for a current block that is to be encoded). Memory 106 ofFIG. 1 may also provide temporary storage of outputs from the variousunits of video encoder 200.

The various units of FIG. 4 are illustrated to assist with understandingthe operations performed by video encoder 200. The units may beimplemented as fixed-function circuits, programmable circuits, or acombination thereof. Fixed-function circuits refer to circuits thatprovide particular functionality, and are preset on the operations thatcan be performed. Programmable circuits refer to circuits that can beprogrammed to perform various tasks, and provide flexible functionalityin the operations that can be performed. For instance, programmablecircuits may execute software or firmware that cause the programmablecircuits to operate in the manner defined by instructions of thesoftware or firmware. Fixed-function circuits may execute softwareinstructions (e.g., to receive parameters or output parameters), but thetypes of operations that the fixed-function circuits perform aregenerally immutable. In some examples, one or more of the units may bedistinct circuit blocks (fixed-function or programmable), and in someexamples, one or more of the units may be integrated circuits.

Video encoder 200 may include arithmetic logic units (ALUs), elementaryfunction units (EFUs), digital circuits, analog circuits, and/orprogrammable cores, formed from programmable circuits. In examples wherethe operations of video encoder 200 are performed using softwareexecuted by the programmable circuits, memory 106 (FIG. 1 ) may storethe instructions (e.g., object code) of the software that video encoder200 receives and executes, or another memory within video encoder 200(not shown) may store such instructions.

Video data memory 230 is configured to store received video data. Videoencoder 200 may retrieve a picture of the video data from video datamemory 230 and provide the video data to residual generation unit 204and mode selection unit 202. Video data in video data memory 230 may beraw video data that is to be encoded.

Mode selection unit 202 includes a motion estimation unit 222, a motioncompensation unit 224, and an intra-prediction unit 226. Mode selectionunit 202 may include additional functional units to perform videoprediction in accordance with other prediction modes. As examples, modeselection unit 202 may include a palette unit, an intra-block copy unit(which may be part of motion estimation unit 222 and/or motioncompensation unit 224), an affine unit, a linear model (LM) unit, or thelike.

Mode selection unit 202 generally coordinates multiple encoding passesto test combinations of encoding parameters and resultingrate-distortion values for such combinations. The encoding parametersmay include partitioning of CTUs into CUs, prediction modes for the CUs,transform types for residual data of the CUs, quantization parametersfor residual data of the CUs, and so on. Mode selection unit 202 mayultimately select the combination of encoding parameters havingrate-distortion values that are better than the other testedcombinations.

Video encoder 200 may partition a picture retrieved from video datamemory 230 into a series of CTUs, and encapsulate one or more CTUswithin a slice. Mode selection unit 202 may partition a CTU of thepicture in accordance with a tree structure, such as the MTT structure,QTBT structure. superblock structure, or the quad-tree structuredescribed above. As described above, video encoder 200 may form one ormore CUs from partitioning a CTU according to the tree structure. Such aCU may also be referred to generally as a “video block” or “block.”

In general, mode selection unit 202 also controls the components thereof(e.g., motion estimation unit 222, motion compensation unit 224, andintra-prediction unit 226) to generate a prediction block for a currentblock (e.g., a current CU, or in HEVC, the overlapping portion of a PUand a TU). For inter-prediction of a current block, motion estimationunit 222 may perform a motion search to identify one or more closelymatching reference blocks in one or more reference pictures (e.g., oneor more previously coded pictures stored in DPB 218). In particular,motion estimation unit 222 may calculate a value representative of howsimilar a potential reference block is to the current block, e.g.,according to sum of absolute difference (SAD), sum of squareddifferences (SSD), mean absolute difference (MAD), mean squareddifferences (MSD), or the like. Motion estimation unit 222 may generallyperform these calculations using sample-by-sample differences betweenthe current block and the reference block being considered. Motionestimation unit 222 may identify a reference block having a lowest valueresulting from these calculations, indicating a reference block thatmost closely matches the current block.

Motion estimation unit 222 may form one or more motion vectors (MVs)that defines the positions of the reference blocks in the referencepictures relative to the position of the current block in a currentpicture. Motion estimation unit 222 may then provide the motion vectorsto motion compensation unit 224. For example, for uni-directionalinter-prediction, motion estimation unit 222 may provide a single motionvector, whereas for bi-directional inter-prediction, motion estimationunit 222 may provide two motion vectors. Motion compensation unit 224may then generate a prediction block using the motion vectors. Forexample, motion compensation unit 224 may retrieve data of the referenceblock using the motion vector. As another example, if the motion vectorhas fractional sample precision, motion compensation unit 224 mayinterpolate values for the prediction block according to one or moreinterpolation filters. Moreover, for bi-directional inter-prediction,motion compensation unit 224 may retrieve data for two reference blocksidentified by respective motion vectors and combine the retrieved data,e.g., through sample-by-sample averaging or weighted averaging.

When operating according to the AV1 video coding format, motionestimation unit 222 and motion compensation unit 224 may be configuredto encode coding blocks of video data (e.g., both luma and chroma codingblocks) using translational motion compensation, affine motioncompensation, overlapped block motion compensation (OBMC), and/orcompound inter-intra prediction.

As another example, for intra-prediction, or intra-prediction coding,intra-prediction unit 226 may generate the prediction block from samplesneighboring the current block. For example, for directional modes,intra-prediction unit 226 may generally mathematically combine values ofneighboring samples and populate these calculated values in the defineddirection across the current block to produce the prediction block. Asanother example, for DC mode, intra-prediction unit 226 may calculate anaverage of the neighboring samples to the current block and generate theprediction block to include this resulting average for each sample ofthe prediction block.

When operating according to the AV1 video coding format, intraprediction unit 226 may be configured to encode coding blocks of videodata (e.g., both luma and chroma coding blocks) using directional intraprediction, non-directional intra prediction, recursive filter intraprediction, chroma-from-luma (CFL) prediction, intra block copy (IBC),and/or color palette mode. Mode selection unit 202 may includeadditional functional units to perform video prediction in accordancewith other prediction modes.

Mode selection unit 202 provides the prediction block to residualgeneration unit 204. Residual generation unit 204 receives a raw,unencoded version of the current block from video data memory 230 andthe prediction block from mode selection unit 202. Residual generationunit 204 calculates sample-by-sample differences between the currentblock and the prediction block. The resulting sample-by-sampledifferences define a residual block for the current block. In someexamples, residual generation unit 204 may also determine differencesbetween sample values in the residual block to generate a residual blockusing residual differential pulse code modulation (RDPCM). In someexamples, residual generation unit 204 may be formed using one or moresubtractor circuits that perform binary subtraction.

In examples where mode selection unit 202 partitions CUs into PUs, eachPU may be associated with a luma prediction unit and correspondingchroma prediction units. Video encoder 200 and video decoder 300 maysupport PUs having various sizes. As indicated above, the size of a CUmay refer to the size of the luma coding block of the CU and the size ofa PU may refer to the size of a luma prediction unit of the PU. Assumingthat the size of a particular CU is 2N×2N, video encoder 200 may supportPU sizes of 2N×2N or N×N for intra prediction, and symmetric PU sizes of2N×2N, 2N×N, N×2N, N×N, or similar for inter prediction. Video encoder200 and video decoder 300 may also support asymmetric partitioning forPU sizes of 2NxnU, 2NxnD, nLx2N, and nRx2N for inter prediction.

In examples where mode selection unit 202 does not further partition aCU into PUs, each CU may be associated with a luma coding block andcorresponding chroma coding blocks. As above, the size of a CU may referto the size of the luma coding block of the CU. The video encoder 200and video decoder 300 may support CU sizes of 2N×2N, 2N×N, or N×2N.

For other video coding techniques such as an intra-block copy modecoding, an affine-mode coding, and linear model (LM) mode coding, assome examples, mode selection unit 202, via respective units associatedwith the coding techniques, generates a prediction block for the currentblock being encoded. In some examples, such as palette mode coding, modeselection unit 202 may not generate a prediction block, and insteadgenerate syntax elements that indicate the manner in which toreconstruct the block based on a selected palette. In such modes, modeselection unit 202 may provide these syntax elements to entropy encodingunit 220 to be encoded.

As described above, residual generation unit 204 receives the video datafor the current block and the corresponding prediction block. Residualgeneration unit 204 then generates a residual block for the currentblock. To generate the residual block, residual generation unit 204calculates sample-by-sample differences between the prediction block andthe current block.

Transform processing unit 206 applies one or more transforms to theresidual block to generate a block of transform coefficients (referredto herein as a “transform coefficient block”). Transform processing unit206 may apply various transforms to a residual block to form thetransform coefficient block. For example, transform processing unit 206may apply a discrete cosine transform (DCT), a directional transform, aKarhunen-Loeve transform (KLT), or a conceptually similar transform to aresidual block. In some examples, transform processing unit 206 mayperform multiple transforms to a residual block, e.g., a primarytransform and a secondary transform, such as a rotational transform. Insome examples, transform processing unit 206 does not apply transformsto a residual block.

When operating according to AV1, transform processing unit 206 may applyone or more transforms to the residual block to generate a block oftransform coefficients (referred to herein as a “transform coefficientblock”). Transform processing unit 206 may apply various transforms to aresidual block to form the transform coefficient block. For example,transform processing unit 206 may apply a horizontal/vertical transformcombination that may include a discrete cosine transform (DCT), anasymmetric discrete sine transform (ADST), a flipped ADST (e.g., an ADSTin reverse order), and an identity transform (IDTX). When using anidentity transform, the transform is skipped in one of the vertical orhorizontal directions. In some examples, transform processing may beskipped.

Quantization unit 208 may quantize the transform coefficients in atransform coefficient block, to produce a quantized transformcoefficient block. Quantization unit 208 may quantize transformcoefficients of a transform coefficient block according to aquantization parameter (QP) value associated with the current block.Video encoder 200 (e.g., via mode selection unit 202) may adjust thedegree of quantization applied to the transform coefficient blocksassociated with the current block by adjusting the QP value associatedwith the CU. Quantization may introduce loss of information, and thus,quantized transform coefficients may have lower precision than theoriginal transform coefficients produced by transform processing unit206.

Inverse quantization unit 210 and inverse transform processing unit 212may apply inverse quantization and inverse transforms to a quantizedtransform coefficient block, respectively, to reconstruct a residualblock from the transform coefficient block. Reconstruction unit 214 mayproduce a reconstructed block corresponding to the current block (albeitpotentially with some degree of distortion) based on the reconstructedresidual block and a prediction block generated by mode selection unit202. For example, reconstruction unit 214 may add samples of thereconstructed residual block to corresponding samples from theprediction block generated by mode selection unit 202 to produce thereconstructed block.

Filter unit 216 may perform one or more filter operations onreconstructed blocks. For example, filter unit 216 may performdeblocking operations to reduce blockiness artifacts along edges of CUs.Operations of filter unit 216 may be skipped, in some examples.

When operating according to AV1, filter unit 216 may perform one or morefilter operations on reconstructed blocks. For example, filter unit 216may perform deblocking operations to reduce blockiness artifacts alongedges of CUs. In other examples, filter unit 216 may apply a constraineddirectional enhancement filter (CDEF), which may be applied afterdeblocking, and may include the application of non-separable,non-linear, low-pass directional filters based on estimated edgedirections. Filter unit 216 may also include a loop restoration filter,which is applied after CDEF, and may include a separable symmetricnormalized Wiener filter or a dual self-guided filter.

Video encoder 200 stores reconstructed blocks in DPB 218. For instance,in examples where operations of filter unit 216 are not performed,reconstruction unit 214 may store reconstructed blocks to DPB 218. Inexamples where operations of filter unit 216 are performed, filter unit216 may store the filtered reconstructed blocks to DPB 218. Motionestimation unit 222 and motion compensation unit 224 may retrieve areference picture from DPB 218, formed from the reconstructed (andpotentially filtered) blocks, to inter-predict blocks of subsequentlyencoded pictures. In addition, intra-prediction unit 226 may usereconstructed blocks in DPB 218 of a current picture to intra-predictother blocks in the current picture.

In general, entropy encoding unit 220 may entropy encode syntax elementsreceived from other functional components of video encoder 200. Forexample, entropy encoding unit 220 may entropy encode quantizedtransform coefficient blocks from quantization unit 208. As anotherexample, entropy encoding unit 220 may entropy encode prediction syntaxelements (e.g., motion information for inter-prediction or intra-modeinformation for intra-prediction) from mode selection unit 202. Entropyencoding unit 220 may perform one or more entropy encoding operations onthe syntax elements, which are another example of video data, togenerate entropy-encoded data. For example, entropy encoding unit 220may perform a context-adaptive variable length coding (CAVLC) operation,a CABAC operation, a variable-to-variable (V2V) length coding operation,a syntax-based context-adaptive binary arithmetic coding (SBAC)operation, a Probability Interval Partitioning Entropy (PIPE) codingoperation, an Exponential-Golomb encoding operation, or another type ofentropy encoding operation on the data. In some examples, entropyencoding unit 220 may operate in bypass mode where syntax elements arenot entropy encoded.

Video encoder 200 may output a bitstream that includes the entropyencoded syntax elements needed to reconstruct blocks of a slice orpicture. In particular, entropy encoding unit 220 may output thebitstream.

In accordance with AV1, entropy encoding unit 220 may be configured as asymbol-to-symbol adaptive multi-symbol arithmetic coder. A syntaxelement in AV1 includes an alphabet of N elements, and a context (e.g.,probability model) includes a set of N probabilities. Entropy encodingunit 220 may store the probabilities as n-bit (e.g., 15-bit) cumulativedistribution functions (CDFs). Entropy encoding unit 22 may performrecursive scaling, with an update factor based on the alphabet size, toupdate the contexts.

The operations described above are described with respect to a block.Such description should be understood as being operations for a lumacoding block and/or chroma coding blocks. As described above, in someexamples, the luma coding block and chroma coding blocks are luma andchroma components of a CU. In some examples, the luma coding block andthe chroma coding blocks are luma and chroma components of a PU.

In some examples, operations performed with respect to a luma codingblock need not be repeated for the chroma coding blocks. As one example,operations to identify a motion vector (MV) and reference picture for aluma coding block need not be repeated for identifying a MV andreference picture for the chroma blocks. Rather, the MV for the lumacoding block may be scaled to determine the MV for the chroma blocks,and the reference picture may be the same. As another example, theintra-prediction process may be the same for the luma coding block andthe chroma coding blocks.

Video encoder 200 represents an example of a device configured to encodevideo data including a memory configured to store video data, and one ormore processing units implemented in circuitry and configured to dividea current block of the video data into a plurality of subblocks;generate initial motion vectors for both prediction directions for eachsubblock according to an initial affine motion model; determine arespective subblock bilateral matching cost for each possible offset ofa plurality of offsets for each subblock; determine a respectiveaccumulated subblock bilateral matching cost for each possible offset,each respective accumulated subblock bilateral matching costcorresponding to a respective bilateral matching cost for the currentblock for one of the possible offsets; based on a correspondingbilateral matching cost for the current block having a lowest bilateralmatching cost of each respective accumulated subblock bilateral matchingcosts, select a possible offset of the possible offsets; and code thecurrent block based on the selected possible offset.

FIG. 5 is a block diagram illustrating an example video decoder 300 thatmay perform the techniques of this disclosure. FIG. 5 is provided forpurposes of explanation and is not limiting on the techniques as broadlyexemplified and described in this disclosure. For purposes ofexplanation, this disclosure describes video decoder 300 according tothe techniques of VVC (ITU-T H.266, under development), and HEVC (ITU-TH.265). However, the techniques of this disclosure may be performed byvideo coding devices that are configured to other video codingstandards.

In the example of FIG. 5 , video decoder 300 includes coded picturebuffer (CPB) memory 320, entropy decoding unit 302, predictionprocessing unit 304, inverse quantization unit 306, inverse transformprocessing unit 308, reconstruction unit 310, filter unit 312, anddecoded picture buffer (DPB) 314. Any or all of CPB memory 320, entropydecoding unit 302, prediction processing unit 304, inverse quantizationunit 306, inverse transform processing unit 308, reconstruction unit310, filter unit 312, and DPB 314 may be implemented in one or moreprocessors or in processing circuitry. For instance, the units of videodecoder 300 may be implemented as one or more circuits or logic elementsas part of hardware circuitry, or as part of a processor, ASIC, or FPGA.Moreover, video decoder 300 may include additional or alternativeprocessors or processing circuitry to perform these and other functions.

Prediction processing unit 304 includes motion compensation unit 316 andintra-prediction unit 318. Prediction processing unit 304 may includeadditional units to perform prediction in accordance with otherprediction modes. As examples, prediction processing unit 304 mayinclude a palette unit, an intra-block copy unit (which may form part ofmotion compensation unit 316), an affine unit, a linear model (LM) unit,or the like. In other examples, video decoder 300 may include more,fewer, or different functional components.

When operating according to AV1, compensation unit 316 may be configuredto decode coding blocks of video data (e.g., both luma and chroma codingblocks) using translational motion compensation, affine motioncompensation, OBMC, and/or compound inter-intra prediction, as describedabove. Intra prediction unit 318 may be configured to decode codingblocks of video data (e.g., both luma and chroma coding blocks) usingdirectional intra prediction, non-directional intra prediction,recursive filter intra prediction, CFL, intra block copy (IBC), and/orcolor palette mode, as described above.

CPB memory 320 may store video data, such as an encoded video bitstream,to be decoded by the components of video decoder 300. The video datastored in CPB memory 320 may be obtained, for example, fromcomputer-readable medium 110 (FIG. 1). CPB memory 320 may include a CPBthat stores encoded video data (e.g., syntax elements) from an encodedvideo bitstream. Also, CPB memory 320 may store video data other thansyntax elements of a coded picture, such as temporary data representingoutputs from the various units of video decoder 300. DPB 314 generallystores decoded pictures, which video decoder 300 may output and/or useas reference video data when decoding subsequent data or pictures of theencoded video bitstream. CPB memory 320 and DPB 314 may be formed by anyof a variety of memory devices, such as DRAM, including SDRAM, MRAM,RRAM, or other types of memory devices. CPB memory 320 and DPB 314 maybe provided by the same memory device or separate memory devices. Invarious examples, CPB memory 320 may be on-chip with other components ofvideo decoder 300, or off-chip relative to those components.

Additionally or alternatively, in some examples, video decoder 300 mayretrieve coded video data from memory 120 (FIG. 1 ). That is, memory 120may store data as discussed above with CPB memory 320. Likewise, memory120 may store instructions to be executed by video decoder 300, whensome or all of the functionality of video decoder 300 is implemented insoftware to be executed by processing circuitry of video decoder 300.

The various units shown in FIG. 5 are illustrated to assist withunderstanding the operations performed by video decoder 300. The unitsmay be implemented as fixed-function circuits, programmable circuits, ora combination thereof. Similar to FIG. 4 , fixed-function circuits referto circuits that provide particular functionality, and are preset on theoperations that can be performed. Programmable circuits refer tocircuits that can be programmed to perform various tasks, and provideflexible functionality in the operations that can be performed. Forinstance, programmable circuits may execute software or firmware thatcause the programmable circuits to operate in the manner defined byinstructions of the software or firmware. Fixed-function circuits mayexecute software instructions (e.g., to receive parameters or outputparameters), but the types of operations that the fixed-functioncircuits perform are generally immutable. In some examples, one or moreof the units may be distinct circuit blocks (fixed-function orprogrammable), and in some examples, one or more of the units may beintegrated circuits.

Video decoder 300 may include ALUs, EFUs, digital circuits, analogcircuits, and/or programmable cores formed from programmable circuits.In examples where the operations of video decoder 300 are performed bysoftware executing on the programmable circuits, on-chip or off-chipmemory may store instructions (e.g., object code) of the software thatvideo decoder 300 receives and executes.

Entropy decoding unit 302 may receive encoded video data from the CPBand entropy decode the video data to reproduce syntax elements.Prediction processing unit 304, inverse quantization unit 306, inversetransform processing unit 308, reconstruction unit 310, and filter unit312 may generate decoded video data based on the syntax elementsextracted from the bitstream.

In general, video decoder 300 reconstructs a picture on a block-by-blockbasis. Video decoder 300 may perform a reconstruction operation on eachblock individually (where the block currently being reconstructed, i.e.,decoded, may be referred to as a “current block”).

Entropy decoding unit 302 may entropy decode syntax elements definingquantized transform coefficients of a quantized transform coefficientblock, as well as transform information, such as a quantizationparameter (QP) and/or transform mode indication(s). Inverse quantizationunit 306 may use the QP associated with the quantized transformcoefficient block to determine a degree of quantization and, likewise, adegree of inverse quantization for inverse quantization unit 306 toapply. Inverse quantization unit 306 may, for example, perform a bitwiseleft-shift operation to inverse quantize the quantized transformcoefficients. Inverse quantization unit 306 may thereby form a transformcoefficient block including transform coefficients.

After inverse quantization unit 306 forms the transform coefficientblock, inverse transform processing unit 308 may apply one or moreinverse transforms to the transform coefficient block to generate aresidual block associated with the current block. For example, inversetransform processing unit 308 may apply an inverse DCT, an inverseinteger transform, an inverse Karhunen-Loeve transform (KLT), an inverserotational transform, an inverse directional transform, or anotherinverse transform to the transform coefficient block.

Furthermore, prediction processing unit 304 generates a prediction blockaccording to prediction information syntax elements that were entropydecoded by entropy decoding unit 302. For example, if the predictioninformation syntax elements indicate that the current block isinter-predicted, motion compensation unit 316 may generate theprediction block. In this case, the prediction information syntaxelements may indicate a reference picture in DPB 314 from which toretrieve a reference block, as well as a motion vector identifying alocation of the reference block in the reference picture relative to thelocation of the current block in the current picture. Motioncompensation unit 316 may generally perform the inter-prediction processin a manner that is substantially similar to that described with respectto motion compensation unit 224 (FIG. 4 ).

As another example, if the prediction information syntax elementsindicate that the current block is intra-predicted, intra-predictionunit 318 may generate the prediction block according to anintra-prediction mode indicated by the prediction information syntaxelements. Again, intra-prediction unit 318 may generally perform theintra-prediction process in a manner that is substantially similar tothat described with respect to intra-prediction unit 226 (FIG. 4 ).Intra-prediction unit 318 may retrieve data of neighboring samples tothe current block from DPB 314.

Reconstruction unit 310 may reconstruct the current block using theprediction block and the residual block. For example, reconstructionunit 310 may add samples of the residual block to corresponding samplesof the prediction block to reconstruct the current block.

Filter unit 312 may perform one or more filter operations onreconstructed blocks. For example, filter unit 312 may performdeblocking operations to reduce blockiness artifacts along edges of thereconstructed blocks. Operations of filter unit 312 are not necessarilyperformed in all examples.

Video decoder 300 may store the reconstructed blocks in DPB 314. Forinstance, in examples where operations of filter unit 312 are notperformed, reconstruction unit 310 may store reconstructed blocks to DPB314. In examples where operations of filter unit 312 are performed,filter unit 312 may store the filtered reconstructed blocks to DPB 314.As discussed above, DPB 314 may provide reference information, such assamples of a current picture for intra-prediction and previously decodedpictures for subsequent motion compensation, to prediction processingunit 304. Moreover, video decoder 300 may output decoded pictures (e.g.,decoded video) from DPB 314 for subsequent presentation on a displaydevice, such as display device 118 of FIG. 1 .

In this manner, video decoder 300 represents an example of a videodecoding device including a memory configured to store video data, andone or more processing units implemented in circuitry and configured todivide a current block of the video data into a plurality of subblocks;generate initial motion vectors for both prediction directions for eachsubblock according to an initial affine motion model; determine arespective subblock bilateral matching cost for each possible offset ofa plurality of offsets for each subblock; determine a respectiveaccumulated subblock bilateral matching cost for each possible offset,each respective accumulated subblock bilateral matching costcorresponding to a respective bilateral matching cost for the currentblock for one of the possible offsets; based on a correspondingbilateral matching cost for the current block having a lowest bilateralmatching cost of each respective accumulated subblock bilateral matchingcosts, select a possible offset of the possible offsets; and decode thecurrent block based on the selected possible offset.

FIG. 6 is a flowchart illustrating example DMVR techniques according toone or more aspects of this disclosure. Video encoder 200 or videodecoder 300 may determine a plurality of subblocks for a current blockof the video data (330). For example, video encoder 200 or video decoder300 may split the current block into a plurality of subblocks.

For each subblock of the plurality of subblocks, video encoder 200 orvideo decoder 300 may a) generate initial motion vectors, the initialmotion vectors including at least one initial motion vector for a firstprediction direction and at least one initial motion vector for a secondprediction direction according to an affine motion model, and b)determine, based on the initial motion vectors, a subblock bilateralmatching cost for each offset among a plurality of offsets (332). Forexample, video encoder 200 or video decoder 300 may, for each subblockof the plurality of subblocks, determine one or more initial motionvectors in the L0 direction and one or more initial motion vectors inthe L1 direction. Video encoder 200 or video decoder 300 may alsodetermine a subblock bilateral matching cost for each possible offset ofthe plurality of offsets, based on the initial motion vectors.

Video encoder 200 or video decoder 300 may, for each respective offsetamong the plurality of offsets, determining a respective summation ofsubblock bilateral matching costs associated with the plurality ofsubblocks thereby generating a plurality of summations of subblockbilateral matching costs (334). For example, video encoder 200 or videodecoder 300 may sum the subblock bilateral matching costs of each of thesubblocks for each offset. Each summation of subblock bilateral matchingcosts among the plurality of summations of subblock bilateral matchingcost may be associated with a respective possible offset.

Video encoder 200 or video decoder 300 may determine a lowest summationof subblock bilateral matching costs from among the plurality ofsummations of subblock bilateral matching costs (336). For example,video encoder 200 or video decoder 300 may determine which of theplurality of summations has a lowest value.

Video encoder 200 or video decoder 300 may select an offset from amongthe plurality of offsets associated with the lowest summation ofsubblock bilateral matching costs to determine a selected offset (338).For example, video encoder 200 or video decoder 300 may select thesummation have the lowest value as the selected offset.

Video encoder 200 or video decoder 300 may modify the affine motionmodel based on the selected offset to generate a modified affine motionmodel (340). For example, video encoder 200 or video decoder may modifyCPMVs by adding and/or subtracting the selected offset to the CPMVs.Video encoder 200 or video decoder 300 may code the current block basedon the modified affine motion model (342). For example, video encoder200 may encode the current block using the modified affine motion modeland video decoder 300 may decode the current block using the modifiedaffine motion model.

In some examples, each summation of the subblock bilateral matchingcosts represents a respective bilateral matching cost for the currentblock. In some examples, a subblock size of the plurality of subblocksincludes at least one of 4×4, 8×8, or 16×16.

In some examples, generating initial motion vectors includes derivingcandidate motion vectors. In some examples, modifying the affine motionmodel based on the selected offset to generate the modified affinemotion model includes modifying, based on the selected offset, a firstmotion vector for the first prediction direction and modifying, based onthe selected offset, a second motion vector for the second predictiondirection.

In some examples, video encoder 200 or video decoder 300 may applyinterpolation to generate motion vector predictors for each of theplurality of offsets. In some examples, applying interpolation includesapplying bilinear interpolation.

In some examples, video encoder 200 or video decoder 300 may estimatesub-pixel offsets using parametric error surface. In some examples,video encoder 200 or video decoder 300 may determine whether a size ofthe current block meets a predetermined threshold, and based on adetermination that the size of the current block meets the predeterminedthreshold, determine the plurality of subblocks.

In some examples, video encoder 200 or video decoder 300 may determinewhether a size of each of the plurality of subblocks will be greaterthan a predetermined threshold, and based on a determination that thesize of each of the plurality of subblocks will be greater than apredetermined threshold, determine the plurality of subblocks. In someexamples, coding includes decoding and video encoder 200 or videodecoder 300 may determine a value of a syntax element signaled in abitstream, the value of the syntax element being indicative ofapplication of decoder side motion vector refinement to the currentblock. In some examples, the affine motion model includes control pointmotion vectors.

FIG. 7 is a flowchart illustrating an example method for encoding acurrent block in accordance with the techniques of this disclosure. Thecurrent block may comprise a current CU. Although described with respectto video encoder 200 (FIGS. 1 and 2 ), it should be understood thatother devices may be configured to perform a method similar to that ofFIG. 7 .

In this example, video encoder 200 initially predicts the current block(350). For example, video encoder 200 may form a prediction block forthe current block. Video encoder 200 may then calculate a residual blockfor the current block (352). To calculate the residual block, videoencoder 200 may calculate a difference between the original, unencodedblock and the prediction block for the current block. Video encoder 200may then transform the residual block and quantize transformcoefficients of the residual block (354). Next, video encoder 200 mayscan the quantized transform coefficients of the residual block (356).During the scan, or following the scan, video encoder 200 may entropyencode the transform coefficients (358). For example, video encoder 200may encode the transform coefficients using CAVLC or CABAC. Videoencoder 200 may then output the entropy encoded data of the block (360).

FIG. 8 is a flowchart illustrating an example method for decoding acurrent block of video data in accordance with the techniques of thisdisclosure. The current block may comprise a current CU. Althoughdescribed with respect to video decoder 300 (FIGS. 1 and 4 ), it shouldbe understood that other devices may be configured to perform a methodsimilar to that of FIG. 8 .

Video decoder 300 may receive entropy encoded data for the currentblock, such as entropy encoded prediction information and entropyencoded data for transform coefficients of a residual blockcorresponding to the current block (370). Video decoder 300 may entropydecode the entropy encoded data to determine prediction information forthe current block and to reproduce transform coefficients of theresidual block (372). Video decoder 300 may predict the current block(374), e.g., using an intra- or inter-prediction mode as indicated bythe prediction information for the current block, to calculate aprediction block for the current block. Video decoder 300 may theninverse scan the reproduced transform coefficients (376), to create ablock of quantized transform coefficients. Video decoder 300 may theninverse quantize the transform coefficients and apply an inversetransform to the transform coefficients to produce a residual block(378). Video decoder 300 may ultimately decode the current block bycombining the prediction block and the residual block (380).

Clause 1A. A method of coding video data, the method comprising:dividing a current block of the video data into a plurality ofsubblocks; generating initial motion vectors for both predictiondirections for each subblock according to an initial affine motionmodel; determining a respective subblock bilateral matching cost foreach possible offset of a plurality of offsets for each subblock;determining a respective accumulated subblock bilateral matching costfor each possible offset, each respective accumulated subblock bilateralmatching cost corresponding to a respective bilateral matching cost forthe current block for one of the possible offsets; based on acorresponding bilateral matching cost for the current block having alowest bilateral matching cost of each respective accumulated subblockbilateral matching costs, selecting a possible offset of the possibleoffsets; and coding the current block based on the selected possibleoffset.

Clause 2A. The method of clause 1A, further comprising refraining fromgenerating a subblock motion field for each possible offset.

Clause 3A. The method of clause 1A or clause 2A, wherein a subblock sizeof the plurality of subblocks comprises 4×4, 8×8, or 16×16.

Clause 4A. The method of any of clauses 1A-3A, wherein determining therespective subblock bilateral matching cost for each possible offset ofthe plurality of offsets for each subblock comprises deriving candidatemotion vectors.

Clause 5A. The method of clause 4A, wherein determining the respectivesubblock bilateral matching cost for each possible offset of theplurality of offsets further comprises adding a respective offset to arespective motion vector in one direction and subtracting the respectiveoffset to a respective motion vector in another direction.

Clause 6A. The method of any of clauses 1A-3A, wherein determining therespective subblock bilateral matching cost for each possible offset ofthe plurality of offsets for each subblock comprises applyinginterpolation to generate motion vector predictors for all possibleoffsets

Clause 7A. The method of clause 6A, further comprising applying bilinearinterpolation for final motion compensation.

Clause 8A. The method of any of clauses 1A-7A, further comprisingestimating sup-pixel offsets using parametric error surface.

Clause 9A. The method of any of clauses 1A-8A, further comprisingdetermining that a size of the current block meets a predeterminedthreshold prior to dividing a current block of the video data into aplurality of subblocks.

Clause 10A. The method of any of clauses 1A-8A, further comprisingdetermining that a size of each of the plurality of subblocks will begreater than a predetermined threshold prior to dividing a current blockof the video data into a plurality of subblocks.

Clause 11A. The method of any of clauses 1A-10A, further comprisingdetermining a value of a syntax element signaled in a bitstream, thevalue of the syntax element being indicative of application of decoderside motion vector refinement to the current block.

Clause 12A. The method of any of clauses 1A-11A, wherein codingcomprises decoding.

Clause 13A. The method of any of clauses 1A-12A, wherein codingcomprises encoding.

Clause 14A. A device for coding video data, the device comprising one ormore means for performing the method of any of clauses 1A-13A.

Clause 15A. The device of clause 14A, wherein the one or more meanscomprise one or more processors implemented in circuitry.

Clause 16A. The device of any of clauses 14A or 15A, further comprisinga memory to store the video data.

Clause 17A. The device of any of clauses 14A-16A, further comprising adisplay configured to display decoded video data.

Clause 18A. The device of any of clauses 14A-17A, wherein the devicecomprises one or more of a camera, a computer, a mobile device, abroadcast receiver device, or a set-top box.

Clause 19A. The device of any of clauses 14A-18A, wherein the devicecomprises a video decoder.

Clause 20A. The device of any of clauses 14A-19A, wherein the devicecomprises a video encoder.

Clause 21A. A computer-readable storage medium having stored thereoninstructions that, when executed, cause one or more processors toperform the method of any of clauses 1A-11A.

Clause 1B. A method of coding video data, the method comprising:determining a plurality of subblocks for a current block of the videodata; for each subblock of the plurality of subblocks: (a) generatinginitial motion vectors, the initial motion vectors comprising at leastone initial motion vector for a first prediction direction and at leastone initial motion vector for a second prediction direction according toan affine motion model, and (b) determining, based on the initial motionvectors, a subblock bilateral matching cost for each offset among aplurality of offsets; for each respective offset among the plurality ofoffsets, determining a respective summation of subblock bilateralmatching costs associated with the plurality of subblocks therebygenerating a plurality of summations of subblock bilateral matchingcosts; determining a lowest summation of subblock bilateral matchingcosts from among the plurality of summations of subblock bilateralmatching costs; selecting an offset from among the plurality of offsetsassociated with the lowest summation of subblock bilateral matchingcosts to determine a selected offset; modifying the affine motion modelbased on the selected offset to generate a modified affine motion model;and coding the current block based on the modified affine motion model.

Clause 2B. The method of clause 1B, wherein each summation of thesubblock bilateral matching costs represents a respective bilateralmatching cost for the current block.

Clause 3B. The method of clause 1B or clause 2B, wherein a subblock sizeof the plurality of subblocks comprises at least one of 4×4, 8×8, or16×16.

Clause 4B. The method of any of clauses 1B-3B, wherein generatinginitial motion vectors comprises deriving candidate motion vectors.

Clause 5B. The method of any of clauses 1B-4B, wherein modifying theaffine motion model based on the selected offset to generate themodified affine motion model comprises modifying, based on the selectedoffset, a first motion vector for the first prediction direction andmodifying, based on the selected offset, a second motion vector for thesecond prediction direction.

Clause 6B. The method of any of clauses 1B-5B, further comprisingapplying interpolation to generate motion vector predictors for each ofthe plurality of offsets.

Clause 7B. The method of clause 6B, wherein applying interpolationcomprises applying bilinear interpolation.

Clause 8B. The method of any of clauses 1B-7B, further comprisingestimating sub-pixel offsets using parametric error surface.

Clause 9B. The method of any of clauses 1B-8B, further comprising:determining whether a size of the current block meets a predeterminedthreshold; and based on a determination that the size of the currentblock meets the predetermined threshold, determining the plurality ofsubblocks.

Clause 10B. The method of any of clauses 1B-9B, further comprising:determining whether a size of each of the plurality of subblocks will begreater than a predetermined threshold; and based on a determinationthat the size of each of the plurality of subblocks will be greater thana predetermined threshold, determining the plurality of subblocks.

Clause 11B. The method of any of clauses 1B-10B, wherein codingcomprises decoding, the method further comprising determining a value ofa syntax element signaled in a bitstream, the value of the syntaxelement being indicative of application of decoder side motion vectorrefinement to the current block.

Clause 12B. The method of any of clauses 1B-11B, wherein the affinemotion model comprises control point motion vectors.

Clause 13B. A device for coding video data, the device comprising:memory configured to store the video data; and one or more processorsimplemented in circuitry and communicatively coupled to the memory, theone or more processors being configured to: determine a plurality ofsubblocks for a current block of the video data; for each subblock ofthe plurality of subblocks: (a) generate initial motion vectors, theinitial motion vectors comprising at least one initial motion vector fora first prediction direction and at least one initial motion vector fora second prediction direction according to an affine motion model, and(b) determine, based on the initial motion vectors, a subblock bilateralmatching cost for each offset among a plurality of offsets; for eachrespective offset among the plurality of offsets, determine a respectivesummation of subblock bilateral matching costs associated with theplurality of subblocks thereby generating a plurality of summations ofsubblock bilateral matching costs; determine a lowest summation ofsubblock bilateral matching costs from among the plurality of summationsof subblock bilateral matching costs; select an offset from among theplurality of offsets associated with the lowest summation of subblockbilateral matching costs to determine a selected offset; modify theaffine motion model based on the selected offset to generate a modifiedaffine motion model; and code the current block based on the modifiedaffine motion model.

Clause 14B. The device of clause 13B, wherein each summation of thesubblock bilateral matching costs represents a respective bilateralmatching cost for the current block.

Clause 15B. The device of clause 13B or clause 14B, wherein a subblocksize of the plurality of subblocks comprises at least one of 4×4, 8×8,or 16×16.

Clause 16B. The device of any of clauses 13B-15B, wherein as part ofgenerating initial motion vectors, the one or more processors areconfigured to derive candidate motion vectors.

Clause 17B. The device of any of clauses 13B-16B, wherein each summationof the subblock bilateral matching costs represents a respectivebilateral matching cost for the current block.

Clause 18B. The device of any of clauses 13B-17B, wherein the one ormore processors are further configured to apply interpolation togenerate motion vector predictors for each of the plurality of offsets.

Clause 19B. The device of clause 18B, wherein as part of applyinginterpolation, the one or more processors are configured to applybilinear interpolation.

Clause 20B. The device of any of clauses 13B-19B, wherein the one ormore processors are further configured to estimate sub-pixel offsetsusing parametric error surface.

Clause 21B. The device of any of clauses 13B-20B, wherein the one ormore processors are further configured to: determine whether a size ofthe current block meets a predetermined threshold; and based on adetermination that the size of the current block meets the predeterminedthreshold, determine the plurality of subblocks.

Clause 22B. The device of any of clauses 13B-21B, wherein the one ormore processors are further configured to: determine whether a size ofeach of the plurality of subblocks will be greater than a predeterminedthreshold; and based on a determination that the size of each of theplurality of subblocks will be greater than the predetermined threshold,determine the plurality of subblocks.

Clause 23B. The device of any of clauses 13B-22B, wherein the one ormore processors are further configured to determine a value of a syntaxelement signaled in a bitstream, the value of the syntax element beingindicative of application of decoder side motion vector refinement tothe current block.

Clause 24B. The device of any of clauses 13B-23B, wherein the affinemotion model comprises control point motion vectors.

Clause 25B. The device of any of clauses 13B-24B, further comprising adisplay configured to display decoded video data.

Clause 26B. The device of any of clauses 13B-25B, further comprising oneor more of a camera, a computer, a mobile device, a broadcast receiverdevice, or a set-top box.

Clause 27B. A computer-readable storage medium having stored thereoninstructions that, when executed, cause one or more processors to:determine a plurality of subblocks for a current block of video data;for each subblock of the plurality of subblocks: (a) generate initialmotion vectors, the initial motion vectors comprising at least oneinitial motion vector for a first prediction direction and at least oneinitial motion vector for a second prediction direction according to anaffine motion model, and (b) determine, based on the initial motionvectors, a subblock bilateral matching cost for each offset among aplurality of offsets; for each possible offset among the plurality ofoffsets, determine a respective summation of subblock bilateral matchingcosts associated with the plurality of subblocks thereby generating aplurality of summations of subblock bilateral matching costs; determinea lowest summation of subblock bilateral matching costs from among theplurality of summations of subblock bilateral matching costs; select anoffset from among the plurality of offsets associated with the lowestsummation of subblock bilateral matching costs to determine a selectedoffset; modify the affine motion model based on the selected offset togenerate a modified affine motion model; and code the current blockbased on the modified affine motion model.

Clause 28B. A device for coding video data, the device comprising: meansfor determining a plurality of subblocks for a current block of thevideo data; means for generating, for each subblock of the plurality ofsubblocks, initial motion vectors, the initial motion vectors comprisingat least one initial motion vector for a first prediction direction andat least one initial motion vector for a second prediction directionaccording to an affine motion model, and means for determining, based onthe initial motion vectors and for each subblock of the plurality ofsubblocks, a subblock bilateral matching cost for each offset among aplurality of offsets; means for determining, for each possible offsetamong the plurality of offsets, a respective summation of subblockbilateral matching costs associated with the plurality of subblocksthereby generating a plurality of summations of subblock bilateralmatching costs; means for determining a lowest summation of subblockbilateral matching costs from among the plurality of summations ofsubblock bilateral matching costs; means for selecting an offset fromamong the plurality of offsets associated with the lowest summation ofsubblock bilateral matching costs to determine a selected offset; meansfor modifying the affine motion model based on the selected offset togenerate a modified affine motion model; and means for coding thecurrent block based on the modified affine motion model.

It is to be recognized that depending on the example, certain acts orevents of any of the techniques described herein can be performed in adifferent sequence, may be added, merged, or left out altogether (e.g.,not all described acts or events are necessary for the practice of thetechniques). Moreover, in certain examples, acts or events may beperformed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors, rather than sequentially.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over as oneor more instructions or code on a computer-readable medium and executedby a hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the techniques described inthis disclosure. A computer program product may include acomputer-readable medium.

By way of example, and not limitation, such computer-readable storagemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage, or other magnetic storage devices, flashmemory, or any other medium that can be used to store desired programcode in the form of instructions or data structures and that can beaccessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if instructions are transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, digital subscriber line (DSL), orwireless technologies such as infrared, radio, and microwave, then thecoaxial cable, fiber optic cable, twisted pair, DSL, or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. It should be understood, however, thatcomputer-readable storage media and data storage media do not includeconnections, carrier waves, signals, or other transitory media, but areinstead directed to non-transitory, tangible storage media. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and Blu-ray disc, wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one ormore DSPs, general purpose microprocessors, ASICs, FPGAs, or otherequivalent integrated or discrete logic circuitry. Accordingly, theterms “processor” and “processing circuitry,” as used herein may referto any of the foregoing structures or any other structure suitable forimplementation of the techniques described herein. In addition, in someaspects, the functionality described herein may be provided withindedicated hardware and/or software modules configured for encoding anddecoding, or incorporated in a combined codec. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinteroperative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A method of coding video data, the method comprising: determining a plurality of subblocks for a current block of the video data; for each subblock of the plurality of subblocks: (a) generating initial motion vectors, the initial motion vectors comprising at least one initial motion vector for a first prediction direction and at least one initial motion vector for a second prediction direction according to an affine motion model, and (b) determining, based on the initial motion vectors, a subblock bilateral matching cost for each offset among a plurality of offsets; for each respective offset among the plurality of offsets, determining a respective summation of subblock bilateral matching costs associated with the plurality of subblocks thereby generating a plurality of summations of subblock bilateral matching costs; determining a lowest summation of subblock bilateral matching costs from among the plurality of summations of subblock bilateral matching costs; selecting an offset from among the plurality of offsets associated with the lowest summation of subblock bilateral matching costs to determine a selected offset; modifying the affine motion model based on the selected offset to generate a modified affine motion model; and coding the current block based on the modified affine motion model.
 2. The method of claim 1, wherein each summation of the subblock bilateral matching costs represents a respective bilateral matching cost for the current block.
 3. The method of claim 1, wherein a subblock size of the plurality of subblocks comprises at least one of 4×4, 8×8, or 16×16.
 4. The method of claim 1, wherein generating initial motion vectors comprises deriving candidate motion vectors.
 5. The method of claim 1, wherein modifying the affine motion model based on the selected offset to generate the modified affine motion model comprises modifying, based on the selected offset, a first motion vector for the first prediction direction and modifying, based on the selected offset, a second motion vector for the second prediction direction.
 6. The method of claim 1, further comprising applying interpolation to generate motion vector predictors for each of the plurality of offsets.
 7. The method of claim 6, wherein applying interpolation comprises applying bilinear interpolation.
 8. The method of claim 1, further comprising estimating sub-pixel offsets using parametric error surface.
 9. The method of claim 1, further comprising: determining whether a size of the current block meets a predetermined threshold; and based on a determination that the size of the current block meets the predetermined threshold, determining the plurality of subblocks.
 10. The method of claim 1, further comprising: determining whether a size of each of the plurality of subblocks will be greater than a predetermined threshold; and based on a determination that the size of each of the plurality of subblocks will be greater than a predetermined threshold, determining the plurality of subblocks.
 11. The method of claim 1, wherein coding comprises decoding, the method further comprising determining a value of a syntax element signaled in a bitstream, the value of the syntax element being indicative of application of decoder side motion vector refinement to the current block.
 12. The method of claim 1, wherein the affine motion model comprises control point motion vectors.
 13. A device for coding video data, the device comprising: memory configured to store the video data; and one or more processors implemented in circuitry and communicatively coupled to the memory, the one or more processors being configured to: determine a plurality of subblocks for a current block of the video data; for each subblock of the plurality of subblocks: (a) generate initial motion vectors, the initial motion vectors comprising at least one initial motion vector for a first prediction direction and at least one initial motion vector for a second prediction direction according to an affine motion model, and (b) determine, based on the initial motion vectors, a subblock bilateral matching cost for each offset among a plurality of offsets; for each respective offset among the plurality of offsets, determine a respective summation of subblock bilateral matching costs associated with the plurality of subblocks thereby generating a plurality of summations of subblock bilateral matching costs; determine a lowest summation of subblock bilateral matching costs from among the plurality of summations of subblock bilateral matching costs; select an offset from among the plurality of offsets associated with the lowest summation of subblock bilateral matching costs to determine a selected offset; modify the affine motion model based on the selected offset to generate a modified affine motion model; and code the current block based on the modified affine motion model.
 14. The device of claim 13, wherein each summation of the subblock bilateral matching costs represents a respective bilateral matching cost for the current block.
 15. The device of claim 13, wherein a subblock size of the plurality of subblocks comprises at least one of 4×4, 8×8, or 16×16.
 16. The device of claim 13, wherein as part of generating initial motion vectors, the one or more processors are configured to derive candidate motion vectors.
 17. The device of claim 13, wherein each summation of the subblock bilateral matching costs represents a respective bilateral matching cost for the current block.
 18. The device of claim 13, wherein the one or more processors are further configured to apply to generate motion vector predictors for each of the plurality of offsets.
 19. The device of claim 18, wherein as part of applying, the one or more processors are configured to apply bilinear interpolation.
 20. The device of claim 13, wherein the one or more processors are further configured to estimate sub-pixel offsets using parametric error surface.
 21. The device of claim 13, wherein the one or more processors are further configured to: determine whether a size of the current block meets a predetermined threshold; and based on a determination that the size of the current block meets the predetermined threshold, determine the plurality of subblocks.
 22. The device of claim 13, wherein the one or more processors are further configured to: determine whether a size of each of the plurality of subblocks will be greater than a predetermined threshold; and based on a determination that the size of each of the plurality of subblocks will be greater than the predetermined threshold, determine the plurality of subblocks.
 23. The device of claim 13, wherein code comprises decode, and wherein the one or more processors are further configured to determine a value of a syntax element signaled in a bitstream, the value of the syntax element being indicative of application of decoder side motion vector refinement to the current block.
 24. The device of claim 13, wherein the affine motion model comprises control point motion vectors.
 25. The device of claim 13, further comprising a display configured to display decoded video data.
 26. The device of claim 13, further comprising one or more of a camera, a computer, a mobile device, a broadcast receiver device, or a set-top box.
 27. A computer-readable storage medium having stored thereon instructions that, when executed, cause one or more processors to: determine a plurality of subblocks for a current block of video data; for each subblock of the plurality of subblocks: (a) generate initial motion vectors, the initial motion vectors comprising at least one initial motion vector for a first prediction direction and at least one initial motion vector for a second prediction direction according to an affine motion model, and (b) determine, based on the initial motion vectors, a subblock bilateral matching cost for each offset among a plurality of offsets; for each possible offset among the plurality of offsets, determine a respective summation of subblock bilateral matching costs associated with the plurality of subblocks thereby generating a plurality of summations of subblock bilateral matching costs; determine a lowest summation of subblock bilateral matching costs from among the plurality of summations of subblock bilateral matching costs; select an offset from among the plurality of offsets associated with the lowest summation of subblock bilateral matching costs to determine a selected offset; modify the affine motion model based on the selected offset to generate a modified affine motion model; and code the current block based on the modified affine motion model.
 28. A device for coding video data, the device comprising: means for determining a plurality of subblocks for a current block of the video data; means for generating, for each subblock of the plurality of subblocks, initial motion vectors, the initial motion vectors comprising at least one initial motion vector for a first prediction direction and at least one initial motion vector for a second prediction direction according to an affine motion model, and means for determining, based on the initial motion vectors and for each subblock of the plurality of subblocks, a subblock bilateral matching cost for each offset among a plurality of offsets; means for determining, for each possible offset among the plurality of offsets, a respective summation of subblock bilateral matching costs associated with the plurality of subblocks thereby generating a plurality of summations of subblock bilateral matching costs; means for determining a lowest summation of subblock bilateral matching costs from among the plurality of summations of subblock bilateral matching costs; means for selecting an offset from among the plurality of offsets associated with the lowest summation of subblock bilateral matching costs to determine a selected offset; means for modifying the affine motion model based on the selected offset to generate a modified affine motion model; and means for coding the current block based on the modified affine motion model. 